Chapter 475: The Role of Circadian Biology in Health and Disease

Circadian rhythms are autonomous, systemic cycles of behavior and physiology that synchronize organismal function at the cell and tissue level in anticipation of the 24-h rotation of the Earth. A common feature of modern “24/7” life is the routine disruption in these evolutionarily conserved endogenous circadian cycles, due to the rise in shift work, jet travel across time zones, exposure to blue light–emitting devices at night, and disrupted sleep. A transformation in understanding the molecular basis of circadian disorders has generated a new wave of research on metabolic disease, inflammation, aging, and cancer. This chapter provides an overview of (1) the basic biology of the circadian system; (2) primary circadian rhythm and interrelated sleep disorders; and (3) the role of the circadian system in both normal human physiology and disease states. Lastly, we review the rapidly emerging field of chronobiology as a pathway for novel diagnostic and therapeutic activity. A glossary of terms used in circadian biology is summarized in Table 475-1.

A daily cycle between light and darkness existed long before the emergence of multicellular life. Eubacteria, the most ancient photosynthetic prokaryote, emerged in the geologic record more than 2.5 billion years ago at the same time as the first endogenous molecular clock. The co-occurrence in molecular evolution of clocks and photosynthesis hints at an interrelated and selective advantage to the clock and energetic processes—indeed clocks coordinate oxygenic reactions with periods of sunlight each day, and perturbation of clock cycles reduces fitness, reproduction, and survival. Additionally, clocks protect photosynthetic organisms from the DNA-damaging effects of sunlight by timing the production of DNA repair processes, such as photolyase-mediated repair, to the nighttime. Across billions of years of evolution, highly conserved circadian clocks (from “circa diem,” “about a day”) have been found in all photosensitive organisms, governing a wide range of behavioral, physiologic, and biochemical processes. A defining property of the circadian clock system is that it enables organisms to anticipate, rather than simply react to, daily changes in the external environment that are tied to the day-night cycle. In mammals, circadian systems are organized hierarchically with a light-responsive “master” circadian pacemaker located within the suprachiasmatic nucleus (SCN) of the anterior hypothalamus, which in turn presides over a network of both extra-SCN and peripheral clocks (see “Anatomic Organization of the Circadian Clock Network,” below). Daily light exposure signals to the SCN and entrains the circadian system to the 24-h day (see “Entrainment and Measurement of the Circadian System,” below), and the SCN in turn maintains synchrony of a diverse network of peripheral clocks via a variety of signals that have as yet to be fully identified, but which involve direct physiologic rhythms (core body temperature), the autonomic nervous system, and neuroendocrine signals, including cortisol as part of the hypothalamic-pituitary-adrenal (HPA) axis.

At the molecular level, mammalian circadian rhythms are generated by a transcription-translation autoregulatory feedback loop. The forward limb of the clock is composed of the basic helix-loop-helix transcription factors (TFs) CLOCK (or its paralogue, NPAS2) and BMAL1, which drive expression of their own repressors (PER and CRY) in the negative limb, in a cycle that repeats itself every 24 h (Fig. 475-1). A second short feedback loop involves CLOCK/BMAL1-mediated transcription of the retinoic acid–related orphan nuclear receptors ROR and REV-ERB, which activate and repress Bmal1 transcription, respectively. Additional posttranslational regulation of the stability and degradation of core clock TFs includes phosphorylation by casein kinase 1 epsilon (CK1ε) and casein kinase 1 delta (CK1δ) and ubiquitination by FBXL3 and FBXL21. In addition to the ~24-h oscillation of core clock genes, a wide array of downstream clock-controlled genes (CCGs) exhibit broad amplitude in expression, ultimately giving rise to rhythmic physiologic processes. The core clock feedback loop and the induction of CCG rhythms also involves epigenetic mechanisms such as acetylation and methylation. The molecular circadian feedback loop is synchronized with sunrise each day by photosensitive melanopsin-expressing neurons within the retina. The retinohypothalamic tract (RHT) represents the input circuit to the SCN that maintains coherent organismal rhythms. Of note, mutations in several of these clock genes are associated with impaired circadian rhythms and physiology in humans (see “Primary Pathologies of the Circadian System,” below). The importance of clock genes in the brain has been demonstrated by genetic studies, which have found that deletion of Bmal1 in the whole brain or regions that span the SCN cause behavioral arrhythmicity, even when genetic ablation occurs late in life. Conversely, restoring Bmal1 expression specifically in brain in adult clock mutant mice can rescue behavioral locomotor rhythms. Of note, whereas CLOCK normally heterodimerizes with BMAL1, NPAS2 is able to functionally substitute for CLOCK within pacemaker neurons; thus while mice lacking either Clock or Npas2 genes maintain rhythmicity, the double mutants lacking both CLOCK and NPAS2 have impaired circadian rhythms of locomotor activity.

A major transformation in our understanding of circadian biology came with the discovery that the molecular clock network is present not only in the SCN, but also within most peripheral tissues, as well as in extra-SCN neurons in the brain. Interestingly, both the SCN and peripheral tissues exhibit a surprisingly large number of cycling transcripts at the genomic level, with approximately 3–16% of the mammalian transcriptome displaying a 24-h variation in mRNA expression levels across any given tissue, although the set of genes exhibiting circadian rhythmicity varies substantially between tissues depending upon tissue-specific functions. Posttranscriptional events such as RNA polyadenylation and mRNA translation also exhibit circadian variation, further increasing the repertoire of circadian processes at a cellular level. Thus, it is not surprising that among genes in the mammalian genome, almost half are considered to display circadian rhythms in at least one tissue.

Understanding the circuit organization of the circadian clock within the brain is increasingly relevant in understanding how the master circadian pacemaker center within the SCN regulates feeding, sleep-wake activity, energy expenditure, endocrine processes, and metabolism (Fig. 475-2). Identification of the SCN as the master pacemaker was first established by the observation that SCN lesioning induced complete loss of rhythms of locomotor activity and endocrine hormone secretion. The ventral “core” region of the SCN, which is composed of neurons producing vasoactive intestinal polypeptide (VIP), receives photic information directly from the retina through the RHT. At the molecular level, circadian gene transcription is induced within the SCN through the initial activation of immediate early genes, such as Per1, Per2, c-fos, and jun. Cells within the “core” region of the SCN then signal primarily via GABA-ergic neurotransmitter release to synchronize the cells within the “shell” region of the SCN, which produce the neuropeptide arginine vasopressin (AVP).

The SCN communicates to extra-SCN and peripheral clocks through both secreted factors and direct neuronal projections. The former was elegantly proven by the ability of SCN grafts to partially restore locomotor rhythms in an SCN-lesioned host. Efferent nerve outputs predominantly arise from the AVP-producing shell region of the SCN, although some output is also derived from the VIP-predominated core. The SCN projects to several hypothalamic relay regions, including the median preoptic nucleus, the subparaventricular zone (SPZ), the dorsomedial hypothalamus (DMH), and the paraventricular nucleus of the hypothalamus (PVH). These, in turn, regulate output to both sleep- and wake-promoting regions, as well as to regions involved in autonomic regulation and feeding. The SCN is thereby thought to promote sleep through the transmission of neural signals that terminate in the sleep-promoting ventrolateral preoptic nucleus (VLPO), i.e., one of the brain regions that is active during sleep. In contrast, the SCN promotes wakefulness during the active phase by transmission of neural signals that—by passing through regions such as the DMH—terminate in wake-promoting regions, including the locus coeruleus, lateral hypothalamic nucleus, ventral tegmental area, and dorsal raphe nucleus.

The SCN also signals via noradrenergic fibers to the pineal gland, thereby regulating the circadian production of the hormone melatonin. SCN control of the nighttime rise in pineal melatonin release (in both diurnal and nocturnal animals) is mediated through a pathway involving the PVH. Of note, artificial light at night is able to delay the secretion of melatonin, ultimately affecting sleep (see “Endocrine Systems Regulated by the Circadian Clock,” below). Melatonin plays a complex role in the circadian system since the MT1 and MT2 melatonin receptors are expressed on the SCN itself; thus melatonin feeds back to modulate circadian outputs to other cells in the brain and body.

Neuronal output from the SCN also reaches the periphery, i.e., to the adrenal glands, the liver, and the pancreas. The SCN produces rhythmic variation in multiple neuroendocrine axes, producing daily rhythms of gonadotropin, thyrotropin, and somatotropin. Prominent hypothalamic-pituitary-adrenal (HPA) axis rhythms ultimately give rise to daily variation in diverse pathways essential for hemodynamic stability, metabolism, and inflammation. These rhythms originate with SCN control of corticotropin-releasing hormone (CRH)-producing cells in the PVH, which induce daily oscillations of both pituitary adrenocorticotropic hormone (ACTH) and adrenal cortisol. Highlighting the importance of SCN output for peripheral rhythms, there is a dramatic reduction in the number of transcripts that exhibit circadian rhythms in the liver following SCN ablation in mice. Nonetheless, when the autonomous clock in the liver is ablated in mice, some key clock transcripts such as Per2 are still able to cycle as long as the core body temperature rhythm persists. Whereas the SCN is exclusively entrained by light, meal timing is able to signal circadian time directly to the liver. Thus, shifted meal timing as occurs during shift work or jetlag can uncouple peripheral clocks from the central pacemaker. In comparison to peripheral tissue clocks, the SCN is also resistant to phase shifts induced by temperature. This is consistent with the concept that the SCN generates the core body temperature rhythm as one of the major mechanisms to signal circadian time to peripheral clocks.

Under normal light-dark cycles, the circadian system is corrected or “entrained” on a daily basis, producing diurnal rhythms of 24 h. Such signals of entrainment are called Zeitgebers (“time-giver” signals) and include light exposure, meal timing, and activity patterns. Light serves as the dominant Zeitgeber for the circadian system, and a breakthrough in understanding photoentrainment in mammals came with the discovery of the melanopsin system, which is composed of a specialized class of photosensitive retinal ganglion cells that expresses the blue-light sensitive photopigment melanopsin in the inner retina, separate from the photoreceptive rods and cones. Blue light around this wavelength (~480 nm) suppresses melatonin and promotes subjective and objective (electroencephalography-assessed) wakefulness.

The ability of light to entrain the circadian system functions according to a so-called phase response curve (PRC). When light exposure occurs prior to the critical phase of the core body temperature (CBT), defined by the CBT’s minimum, light produces a phase delay in the circadian rhythm. Conversely, light exposure after this critical period causes phase advances. The circadian system can respond even to small changes in light intensity (e.g., dim light at ~100 lux can produce half of the phase delay compared with an almost 100-fold greater light exposure). This responsiveness is furthermore affected by our genetic makeup, as variants in clock genes can modulate the response of the human circadian system to light.

When an organism is placed in an environment without Zeitgebers, the circadian rhythm is said to free-run, as it will rely on the endogenous rhythm of the circadian system. In humans, the study of endogenous circadian rhythms can be achieved by using a so-called constant routine. In these paradigms, subjects are maintained in a constant semi-recumbent posture, meals are provided on an hourly basis, light is constantly kept below the level which phase shifts the SCN, and circadian rhythms are assessed by measuring CBT, melatonin, or peptidergic hormone rhythms. In animals, circadian rhythms are instead studied by examining behavior and physiologic responses after 30–36 h of complete darkness, and endogenous rhythms are assessed by measuring voluntary locomotor activity. From these measurements, key properties of the circadian system can be ascertained, such as period length (peak-to-peak or trough-to-trough time), amplitude (peak-to-trough difference) and phase (timing of peak or trough in relation to a reference point) (Fig. 475-2).

These studies have revealed that the endogenous human circadian clock runs with a period length of approximately 24.2 h (compared with mice that run at ~23.5 h, depending on the strain). Evidence indicates that human females may have a slightly shorter circadian clock (24.1 vs 24.2 h). Notably, interindividual variability in the endogenous circadian period length is further diversified by the existence of genetic polymorphisms in clock genes (see below). These gene variants can confer extremes in endogenous circadian period as well as phase; the latter can be advanced or delayed by about 3–4 h in each direction. This is due both to altered circadian rhythms at the cell level and to altered SCN responsiveness to entrainment by light. For instance, PER3 exists in a variable-number, tandem-repeat polymorphism. Individuals homozygous for a PER3 5/5 genotype have been reported to be more responsive than PER3 4/4 homozygous individuals to the melatonin-suppressing effect of evening blue light exposure.

Using specifically developed questionnaires to establish preferred sleep-wake timing, individuals can be categorized into so-called morningness-eveningness types or chronotypes. The most commonly used questionnaires are the Horne-Östberg morningness-eveningness questionnaire (MEQ) and the Munich ChronoType Questionnaire (MCTQ). The MEQ is composed of 19 questions regarding traits distinguishing morning-type compared with evening-type individuals, such as preferred waking time. In contrast, the MCTQ centers on the midpoint of sleep as a circadian marker, queries age and sex across a range of geographical locations, and can be used to ascertain differences between socially imposed sleep patterns (e.g., on working days) and sleep patterns on free days (the difference constituting so-called social jetlag). According to MCTQs obtained from primarily European populations, ~1% of the general population goes to bed before 10:00 PM and about 8% after 03:00 AM. Differences in chronotype are linked to altered circadian timing—including peak levels of melatonin, which can vary by up to 4 h between extreme morning and evening types. Extreme chronotypes have also been shown to be linked to various traits; i.e., low morningness scores have been associated with greater tolerance to shift work.

Melatonin is one of the most commonly used peripheral markers of an individual’s circadian rhythm, reflecting the rhythmic function of the SCN. Circadian rhythms of melatonin can be measured in saliva or plasma, while 6-sulphatoxymelatonin (aMT6S), a metabolite generated from the breakdown of melatonin, can also be measured in urine. Accurate estimations of melatonin rhythms are often obtained by analyzing the dim light melatonin onset (DLMO), which as the name implies does not require an entire 24-h sampling, making this marker useful in both the clinical and research setting. In normally entrained individuals, the DLMO can be used to ascertain whether an individual’s circadian rhythm is phase advanced or delayed, and this onset typically occurs ~2 h before the onset of sleep. The midpoint of sleep—the main marker used by the MCTQ—also strongly correlates with melatonin onset.

The CBT is also often utilized as an indicator of the circadian rhythm, and even though the outcome is more variable when using the CBT, it usually correlates well with the phase obtained using the melatonin rhythm. The CBT, however, can be masked by factors such as sleep, food intake, and activity. CBT can be recorded and registered wirelessly with relative ease. In humans, CBT can for example be recorded via rectal thermometers or probes that are swallowed to pass through the gastrointestinal tract. When humans are studied under normal conditions with normal lighting and sleep duration from 2300 to 0700 h, the CBT reaches around 37.2°C by 0900 h, and from there continues to rise slowly until it reaches 37.4°C around 11 h later. The CBT then drops to the daily low of 36.5°C in the early morning (0400 h).

Given the interrelationship between the circadian system and sleep-wake systems, researchers have developed paradigms that uncouple the circadian system from sleep-wake states, enabling the study of the contribution of the circadian system to investigated parameters across the entire sleep-wake cycle. These paradigms are known as “forced desynchrony” protocols and involve enforcing a significantly shortened (e.g., 20 h) or prolonged (e.g., 28 h) day length upon individuals. These protocols thus attempt to approximate what occurs during rotating shift work or “jetlag,” e.g., when travel across several time zones suddenly shifts the light-dark and behavioral cycle drastically away from the entrained 24-h rhythm. As described below, forced desynchrony protocols have contributed to uncovering how the circadian system regulates parameters such as cognitive performance, subjective alertness, and metabolic and cardiovascular health.

An overarching term for disorders of the circadian system is circadian rhythm sleep disorders (CRSD) (See Also Chap. 27). These disorders have become increasingly recognized as important factors in a number of conditions; a unifying feature of CRSD involves a mismatch between subjective behavioral and physiologic rhythm with the environmental light-dark or social activity-rest cycle (i.e., the body clock is out of sync with the external light-dark cycle). CRSDs can arise either due to misalignment of an exogenous environmental factor, such as light, with the intrinsic circadian cycle, or due to misalignment of activity/rest cycle in relation to endogenous circadian timing (e.g., shift work or jetlag). In addition to such environmental or exogenous conditions causing circadian disruption, in some cases intrinsic circadian timing is altered in relation to the external environment, as in the case of endogenous circadian disorders that include those caused by mutations in core clock genes. Under conditions of intrinsic circadian abnormalities, it is often exceedingly difficult for individuals suffering from CRSDs to try to properly realign themselves, and these disorders often result in adverse effects such as sleepiness or depressed mood. Societal and economic consequences also are common; these can result in the individual being unable to maintain a job or be unable to attend at regular school hours. The criteria for CRSDs based on the International Classification of Sleep Disorders (ICSD) is shown in Table 475-2.

Animal models have greatly advanced our understanding of how core molecular clock components contribute to maintaining normal sleep-wake/rest-activity cycles (Table 475-3). For example, Clock^Δ19/Δ19 mice have reduced total sleep duration and less induction of REM sleep in response to sleep deprivation. Further, mice that lack Bmal1 have increased total sleep time, but it is more fragmented and lacks clear 24-h sleep-wake rhythms, and mice lacking the repressors Cry1 and Cry2 are not only arrhythmic but spend more time in non-REM sleep. Finally, while ablation of the circadian gene Dbp does not alter the specific duration of sleep stages, it does lead to an altered circadian sleep-wake distribution, with more sleep during their normal wake period and vice versa. Consistent with a key role of clock genes in regulating sleep-wake behavior, human genetic studies of twins have found that up to half of the variation in diurnal preference is heritable. Established genetic variants associated with diurnal preference and circadian sleep disorders are listed in Table 475-4.

Delayed Sleep Phase Disorder

Delayed sleep phase disorder (DSPD) is one of the more common circadian rhythm sleep disorders and is characterized by chronic and significant delays in both sleep onset and wake times compared to normal “socially acceptable” sleep-wake hours (i.e., scoring as “extreme night owls” on morningness-eveningness preference tests). Rhythms of CBT and melatonin levels in plasma and urine are likewise delayed and the circadian period (tau) is longer in DSPD. Onset of DSPD most commonly occurs during adolescence or early adulthood. While the precise etiology of DSPD is not well established, it has been associated with polymorphisms within the circadian clock genes CLOCK and PER3. An integrated behavioral and pharmaceutical therapeutic approach has been found to be most effective at treating individuals with DSPD. Such treatments include a combination of bright-light therapy soon after waking in the morning (and/or dark-room therapy in the evening) and melatonin administration in the evening several hours prior to the onset of sleep. These approaches aim to realign endogenous circadian rhythms with the desired sleep-wake schedule. As individuals suffering from DSPD also phase delay more rapidly, this explains why attempts to phase advance their sleep schedule can be difficult, as well as why relapse can easily occur after initial treatment.

Advanced Sleep Phase Disorder

Another circadian rhythm sleep disorder whereby one gets the correct amount and quality of sleep but at a shifted time is advanced sleep phase disorder (ASPD). Individuals with ASPD experience an advance in their major sleep episode in relation to the desired clock time. Thus, this disorder typically results both in very early evening bedtimes and morning awakenings (e.g., “extreme early birds”), resulting in reduced quality of life due to excessive sleepiness during early evening, even in social situations. Individuals with ASPD also have phase-advanced temperature and melatonin rhythms in parallel with their earlier sleep onsets. ASPD occurs more often in older individuals, although early-onset autosomal dominant familial variants (familial advanced sleep phase syndrome [FASPS]) have also been associated with mutations in either the PER2 or the casein kinase 1δ (CK1δ) genes. PER2 is critical for SCN resetting by light, and the identification of PER2 mutations in familial ASPD was the first in which clock genes were tied to a CRSD. Such mutations have been found to be able to shorten the endogenous circadian period to about 23.3 h compared with the normal 24.2-h period length. Accordingly, ASPD can be distinguished from other non-circadian sleep disorders by an early onset of dim-light melatonin secretion, a reliable marker for the timing of endogenous circadian rhythms both in the research laboratory as well as in the clinical setting. Polysomnography (PSG) or actigraphy are not required for diagnosing ASPD, although actigraphy may be significantly more feasible for long-term analysis of circadian timing of sleep. Treatments for ASPD include bright light or blue-enriched phototherapy in the evening hours to delay the phase of the circadian clock to a later hour.

Non-24-h Sleep-Wake Rhythm Disorder

Individuals with non-24-h sleep-wake rhythm disorder (“non-24”), otherwise known as free-running disorder (FRD), have endogenous circadian rhythms that are not synchronized with the external 24-h day-night cycle due to an inability to readjust the circadian clock to the 24-h day on a daily basis. This most commonly occurs in individuals who are completely blind (i.e., lacking all photoreceptors) since they are unable to respond to daily light cues, which normally would reset the endogenous circadian clock on a daily basis (although the condition has also been reported in sighted individuals). Instead, the sleep-wake period length corresponds to the individual’s endogenous circadian rhythms, which are typically slightly longer than 24 h, thereby shifting sleep and wake cycles over time in relation to the light-dark cycle. Instead of sleeping at the same time each day, their sleep time would gradually be delayed each day until their sleep period literally goes “around the clock.” Depending on the individual’s endogenous rhythm, the individual will take a given number of days to re-align his or her endogenous phase (in a 360^° phase plot) with the zero time point in the exogenous 24-h light-dark cycle. For example, if an individual has an endogenous 24.5-h rhythm, it would take that individual 48 days to cycle from one cycle to the next. Because of this chronic cycling, prominent symptoms of non-24 include sleep-wake cycle disruption (insomnia and daytime sleepiness), impaired alertness and mood levels, and severe difficulties partaking in normally scheduled work, school, or social activities. Non-24 can be diagnosed following diurnal analysis of an individual’s melatonin or cortisol rhythms, in combination with analyses of sleep diaries where the sleep onset and offset can be visualized over time to identify the free-running period. Treatments for sighted non-24-h patients include a combination of bright light therapy with appropriately timed melatonin administration, while melatonin and dual melatonin (MT1 and MT2) receptor agonist administration in completely blind non-24-h patients has been shown to entrain free-running rhythms and improve symptoms.

Shift Work Sleep Disorder

Given the increased prevalence of shift work in today’s 24/7 society and the accumulating evidence for increased incidence of sleep and metabolic disorders, including obesity, diabetes, cardiovascular disease, and cancer, in shift workers, the need to develop effective treatments for shift work sleep disorder (SWSD) are increasingly important. Shift work sleep disorder is at its core defined by the primary symptom of either insomnia or excessive sleepiness, occurring as a transient phenomenon in relation to work that usually is scheduled during the habitual hours of sleep or comprises irregular work hours. The symptoms may result from recovery of sleep having to consume a large proportion of the individual’s free time, which may produce negative social consequences such as difficulties maintaining social relationships. Older individuals are typically at an increased risk of SWSD due to age-associated decline in the ability to maintain sleep during normal waking hours. Since the symptoms likely arise from a misalignment of sleep-wake rhythms with the external light-dark cycle, therapeutic approaches aim to realign endogenous circadian rhythms with the sleep cycles dictated by work. In addition to optimizing the sleep environment at home to minimize disruptions, timed bright light therapy can help individuals with SWSD—i.e., for night workers, intermittent bright light exposure during the night and avoidance of bright light during the morning, even on days off, has been shown to improve sleep and feelings of alertness. Melatonin prior to bedtime may also help improve symptoms of SWSD. Genetic screening combined with chronotype questionnaires may become useful tools for determining whether a given individual is suited for shiftwork. For instance, a twin study indicated that a genetic variant of the circadian gene DEC2 was associated with reduced sleep duration, and with shorter recovery sleep following extended sleep deprivation. More studies may reveal additional genetic variants that confer an advantage to repeated phase advances and phase delays as typically occurs in shift work.

Irregular Sleep-Wake Rhythm

Damage to the SCN can produce arrhythmicity in animals and is thought to be one of the possible underlying reasons for the temporally disorganized sleep-wake pattern that characterizes the disorder known as irregular sleep-wake rhythm (ISWR). Other contributing factors may be a reduced responsiveness to entraining signals such as light and physical activity, as well as a decreased exposure to such signals as often occurs with increasing age due to, for example, increased risk of poor health and impaired mobility. Whereas the total sleep time per 24 h may be comparable, there is a relative absence of a circadian pattern to the sleep-wake cycle. Sleep timing throughout the sleep-wake cycle can be shortened—sometimes close to randomly distributed—instead of occurring in several distinct bouts. ISWR is often associated with neurologic impairment, foremost Alzheimer’s disease in older age; however, ISWR can also occur in individuals with poor sleep hygiene. The most effective treatments for ISWR involve not pharmacotherapy but rather multimodal interventions such as increased light exposure, improved sleep hygiene, and promotion of social and physical activities.

Jetlag

Most have experienced symptoms associated with jetlag, including insomnia, day-time sleepiness, and fatigue, when traveling from one time zone to another, as one’s endogenous circadian rhythms are not yet aligned, or entrained, to the new external light-dark cycle. This is due to the slowness of the circadian system to adapt to the new time zone: typically, the human circadian system is able to shift ~1.5 h a day in the westward direction (i.e., a phase delay), whereas it shifts more slowly (about 1 h daily) with eastward direction of travel (i.e., achieving a phase advance). Importantly, the symptoms are distinguished from the more short-lived symptoms that can partially result from exposure to traditional airplane cabin conditions, including abdominal distention, dependent edema, muscle cramps, headaches, nausea, and, intermittent dizziness. Usually symptoms of jetlag abate within the first couple of days after traveling, and may present themselves after a first night of good sleep (which is more dependent on a high build-up of homeostatic sleep pressure). Older individuals (age >50) appear to be more at risk. While symptoms are transient, therapeutic approaches can alleviate or temper some of the side effects of travel by hastening synchronization of the internal and external circadian cycles. Behavioral treatments include appropriately timed bright-light exposure and avoidance of bright light during the night time in the new destination, while pharmacologic approaches include timed melatonin administration before bedtime both prior to and following travel, resulting in improved sleep quality and decreased night waking.

Social Jetlag

Individuals with a late chronotype are prone to suffer from “social jetlag,” a phenomenon in which individuals are forced to awaken at a point at which their bodies are entrained to be asleep due to discrepancy between alignment of social and biological time. Social jetlag can be estimated using questionnaires, such as the MCTQ, to compare sleep timing on non-free compared with free days. This has established that a large proportion of the European population suffers from 2 or more hours of social jetlag. Chronic social jetlag has been associated with an increased risk of developing obesity and the metabolic syndrome, as well as with greater alcohol consumption and smoking.

The aforementioned categories of defined clinical circadian disorders have been traditionally established based upon consideration of the endogenous behavioral and physiologic cycles (primarily of melatonin and temperature) with the external 24-h light-dark cycle. In the following sections, we build on the concepts of circadian behavioral disorders to consider new and emerging insight into the role of circadian disruption in organismal homeostasis (Figs. 475-3 and 475-4), and the availability of genetic strategies to dissect the interrelationship between clock function, health, and disease.

Endocrine Systems Regulated by the Circadian Clock

In addition to regulation of behavioral rhythms such as sleep/wake and fasting/feeding cycles, the circadian clock also regulates rhythms of the endocrine system. Cortisol rhythms are regulated through a feedback loop known as the hypothalamic-pituitary (HPA) axis (Chap. 379). Hypothalamic secretion of CRH and AVP promotes secretion of pituitary adrenocorticotropic hormone (ACTH), which in turn regulates rhythmic cortisol secretion from the adrenal cortex. Cortisol release increases towards the morning, and this increase is believed to prepare the brain and peripheral tissues for daytime activity and food intake. Daytime sleep can blunt circulating cortisol levels, presumably through the occurrence of non-REM sleep. AVP secretion in mice occurs prior to sleep to promote water intake, thereby preventing dehydration during the sleep period. Several hormonal systems are in fact influenced more by sleep than by circadian rhythms. For instance, secretion of growth hormone (GH) is profoundly blunted during acute overnight wakefulness. The secretion of this hormone is primarily dependent on the occurrence of slow-wave sleep, which is a homeostatically driven sleep stage that occurs primarily in the first part of the sleep period. Cortisol also exhibits a peak close after wakefulness: the cortisol awakening response (CAR). This peak seems to be independent of a circadian rhythm, as the CAR is severely blunted by acute overnight wakefulness. Curtailed sleep and overnight wakefulness increase the activity of the HPA axis and may increase diurnal cortisol levels. Sleep also influences melatonin amplitude, such that sleep deprivation can increase melatonin levels. In working environments, the effects of curtailed sleep are often confounded by mistimed exposure to light. Even low levels of light are able to potently suppress melatonin secretion. Together with altered timing in light exposure, perturbed hormonal levels may represent a mechanism through which altered timing and duration of sleep may impact central and peripheral circadian oscillators.

Centrally controlled rhythms of melatonin and cortisol are considered key regulators of extra-SCN and peripheral oscillators. Glucocorticoid receptors exist in both the central nervous system and in peripheral tissues such as skeletal muscle, liver, and adipose tissue. Following acute shifts in light-dark or feeding cycles, 24-h rhythms of circulating cortisol appear to shift more slowly than other rhythms and may thus contribute to adverse effects of circadian misalignment by hampering proper realignment of peripheral clocks. Glucocorticoids shift clock gene expression in muscle, kidney, and lung, while the powerful synthetic glucocorticoid dexamethasone is able to synchronize (e.g., reset) circadian rhythms of cells in culture, including liver cells. Consistent with a role for glucocorticoid regulation of the clock, both adrenalectomy, which results in a lack of cortisol, and exogenous corticosteroid supplementation significantly disrupt the circadian clock system.

Several peripherally produced hormones and peptides are not only produced rhythmically but can also feedback to central clocks, including the SCN. For instance, both cortisol and thyroid hormones regulate their own rhythmic synthesis by feedback to central brain regions, i.e., the hypothalamus (for cortisol) and pituitary (for both hormones). Several other peripherally produced factors have been proposed to influence the central clock, including fatty acids produced by the adipose tissue and fibroblast growth factor 21, a hormone primarily produced by the liver. Peripheral hormones that signal energy state and hunger also exhibit circadian rhythms. The most extensively studied hormones are leptin, which is released from white adipose tissue cells, and ghrelin, which is released from the upper fundus region of the stomach. Ghrelin also exhibits significant peaks related to anticipated meal timing, which persist for several days of fasting in humans. Circulating rhythms of leptin and ghrelin are disrupted in circadian mutant mice and are also perturbed in humans subjected to circadian misalignment. For instance, Per and Cry mutant mice exhibit severely blunted leptin rhythms, and wild-type mice exposed to jetlag—through repeatedly altered light-dark cycles—show a reduced wake-associated decrease in leptin. Similarly, humans forced to live 28-h days exhibit increased 24-h profiles of ghrelin, and conversely decreased levels of leptin. Ghrelin and leptin signal to several regions of the brain, including integrative appetitive regions of the hypothalamus such as the arcuate and paraventricular region. Through actions in several such central sites, these hormones influence rhythms of food intake and energy homeostasis in a nutrient-dependent manner.

Role for the Clock in Metabolic Homeostasis

Circadian control of glucose homeostasis has long been recognized, as early studies demonstrated variation in glucose tolerance and insulin action across the day. For example, oral glucose tolerance is impaired in the evening and afternoon compared with the morning due to a combination of circadian control of both peripheral insulin sensitivity and pancreatic β-cell insulin secretion. Another example is the “dawn phenomenon,” whereby glucose levels peak prior to the onset of activity. Further, destruction of the SCN has been shown to abolish circadian regulation of glucose metabolism in rats, and daily cycles of insulin secretion and glucose tolerance are often perturbed in patients with type 2 diabetes. Changes have also been observed in first-degree relatives of patients with type 2 diabetes, possibly highlighting a key hereditary component of the circadian clock in the pathogenesis of type 2 diabetes.

Ablating clock genes in mice has revealed a key function for both central and peripheral clocks in regulating energy homeostasis. The circadian system has been shown to regulate rhythmic insulin secretion from the pancreas via both neural signals and hormonal levels (e.g., cortisol and norepinephrine), as well as via cell autonomous clock regulation within the pancreatic β-cell itself. An early observation was that whole-body mutant Clock^Δ19/Δ19 mice developed obesity without displaying hyperinsulinemia, a phenomenon that indicated concurrent β-cell failure. This was later confirmed using pancreas- and β-cell–specific Bmal1-deficient mice, which exhibited glucose intolerance, hypoinsulinemia, and impaired glucose-stimulated insulin secretion. The molecular clock within other peripheral tissues such as liver, adipose tissue, and skeletal muscle also regulate circadian fluctuations in insulin sensitivity and glucose disposal, which are highest in the morning and decline towards the evening. Liver-specific Bmal1 mutant studies have revealed liver clock promotion of gluconeogenesis, glycogenolysis, and mitochondrial oxidative metabolism in the sleep/fasting period while promoting glycogen synthesis in the wake/feeding period. Muscle-specific Bmal1 deficient mice display reduced glucose tolerance, concomitant with lower levels of proteins involved in glucose uptake by muscle cells (e.g., the glucose transporter GLUT4). Ablation of the Cry1 and Cry2 repressors in the negative limb of the clock alters glucagon and glucocorticoid signaling in the liver, contributing to hyperglycemia and impaired glucose tolerance in these mutant mice. Together, these genetic studies in mice suggest a role for tissue-specific clocks in the partitioning of energy utilization across the sleep-wake cycle.

Importantly, peripheral clocks also interact with other environmental factors such as diet and time of feeding. For example, high-fat feeding leads not only to obesity and metabolic syndrome in mice, but also to perturbed clock gene expression across multiple peripheral tissues and a disrupted sleep-wake/fasting-feeding cycle, as revealed by increased activity and feeding during the daytime. Furthermore, mice that are fed a high-fat diet exclusively during the light phase gain significantly more weight than mice that are fed the same diet during the dark period—the active period for mice. Additionally, the metabolic phenotypes arising from ad lib high-fat feeding can be significantly ameliorated by restricting the time of high-fat feeding exclusively to the dark period. Time-restricted feeding can also increase the activity of brown adipose tissue in mice and reduce hepatic glucose production to instead promote beta oxidation of fatty acids. These findings have been corroborated in human interventional studies, which have demonstrated that time-restricted feeding can improve metabolic homeostasis and promote weight loss. Time-restricted feeding may also modulate central regulation of sleep and hunger, as a study found that humans who restricted their food intake to a shorter than ad lib period also consumed less daily calories and reported improved sleep.

Finally, animal studies have further shown that when the light-dark cycle is disrupted or animals are subjected to conditions mimicking “jetlag”—by artificially advancing or delaying the daily light period—there is desynchronization amongst circadian clocks and subsequent weight gain. Accumulating evidence in humans suggests that circadian misalignment both disrupts and desynchronizes circadian clocks across tissues. Prolonged circadian misalignment using forced desynchrony protocols reduces insulin sensitivity in the pre- and postprandial state. Under such conditions, insulin secretion fails to suppress glucose levels, suggesting inadequate β-cell compensation. Moreover, resting metabolic rate declines significantly both in the awake and sleeping state, altogether providing potential explanations why shift work can increase the risk of obesity, type 2 diabetes, and the metabolic syndrome.

Human genetic association studies also support a role for clock genes in metabolic homeostasis and beta cell function. Carriers of a certain BMAL1 polymorphism have a greater risk of developing type 2 diabetes, while CLOCK variants have been found to interact with diet, such that variants can have a protective effect on insulin sensitivity in individuals with high monounsaturated fat intake or in individuals provided a low-fat diet. Instead, the minor allele of another CLOCK gene variant has been associated with increased waist circumference, but only in those with high saturated fat intake. Similarly, NPAS2 and BMAL1 variants have been associated with a greater risk of hypertension. Melatonin receptor MTNR1B gene variants, which result in increased expression of MTNR1B, have been associated with elevated fasting blood glucose levels and reduced insulin secretion irrespective of their level of glycemic control, consistent with the known effect of melatonin on insulin secretion and lower insulin secretion in the evening. These association studies highlight the role of the circadian system in metabolism, as well as potential for interactions of external perturbations—such as circadian misalignment—with a protective or adverse genetic profile.

A large proportion of society recurrently shifts sleep-wake times between working/non-free days and free days. This social jetlag has been increasingly tied to metabolic disruptions, including a greater risk of obesity and type 2 diabetes. As this involves recurrent phase advances and phase delays—like shift work but of smaller magnitude—it is possible that social jetlag also results in perturbed energy expenditure, in combination with disruptions to the circadian rhythm of hunger drive, further increasing the risk of obesity. Repeated shifts in the food- and SCN-driven rhythm of insulin release may similarly over time increase the risk of type 2 diabetes. Shifted feeding rhythms in relation to the sleep-wake cycle and the timing of SCN activity may be causally involved in this pathogenesis. This is exemplified by the disorders known as night-eating syndrome and sleep-related eating disorder. In the former syndrome, a large part of daily calorie consumption occurs in the evening and nighttime hours, and this shifted meal pattern has been associated with a delayed timing of the internal clock. Some evidence exists that these syndromes are associated with obesity. Individuals who report sleeping fewer hours or who are subjected to restricted sleep for a few consecutive days have also been found to consume more calories later in the evening, perhaps explaining why sleep restriction increases the risk of obesity. These associations have also been observed in individuals with later onset of sleep, i.e., evening chronotypes. Night-eating syndrome and later chronotypes have also been linked to type 2 diabetes and may be more common than other eating disorders such as binge-eating disorder. Both conditions have also been found to be associated with impaired glycemic control—such as a greater likelihood of hemoglobin A1c values exceeding 7%—in patients already suffering from type 2 diabetes, emphasizing how proper alignment of internal circadian rhythms with external factors are key contributing factors for long-term metabolic homeostasis.

Circadian Clocks in Relation to Brain Health and Cognition

Molecular circadian clocks are present not only within the extra-SCN regions of the brain but also in neurons, astrocytes, and microglia. Emphasizing the functional significance of properly aligned clocks for brain health, shift workers have been found to have decreased grey matter in brain regions involved in memory and executive functions, with more notable effects in individuals who had shorter recovery periods between the onset of each shift work cycle. Adults performing rotating shift work for many years have also been shown to exhibit signs of accelerated cognitive aging. Notably, evidence suggests that these effects may be reversible, as those who have stopped carrying out shift work exhibit normal cognitive performance 5 or more years later.

Recent studies have also uncovered an important role for perturbed circadian and sleep-wake rhythms in neurodegenerative conditions such as Alzheimer’s disease (AD), Huntington disease (HD), and Parkinson’s disease (PD). Amyloid beta, a key pathognomonic component of AD, normally exhibits circadian fluctuations in the extracellular space in the brain, as well as in the cerebrospinal fluid and plasma in humans, peaking during the active period and falling during sleep. Of note, these daily rhythms of amyloid beta accumulation are dampened in mice that are prone to develop AD; reduced fluctuations in plasma amyloid beta fluctuations have also been noted in older compared with younger individuals. It is believed that removal of amyloid beta (and other neurotoxic substances) during the nighttime sleep period is facilitated by a lymphatic-like system that relies on glial cells (the “glymphatic” system). Whereas the function of this system has been shown to be important in mice, its circadian components and relevance to humans remain to be determined. Consistent with a role for circadian rhythms in the pathogenesis of AD, ablation of core clock genes throughout the brain or within subregions of the brain increases oxidative stress and neuronal cell death, while promoting scarring of brain tissue (astrogliosis). Furthermore, perturbed light-dark cycles increased pathology associated with oxidative stress, and single-nucleotide polymorphisms in Clock and Bmal1 have been associated with increased risk of developing AD.

Evidence also indicates that the relationship between the circadian/sleep-wake system and AD is bidirectional. For example, patients suffering from AD exhibit several signs of perturbed circadian rhythms, the most prominent of such phenomena being “sundowning,” whereby AD patients become more agitated and exhibit delirium-like symptoms in the afternoon or evening. Studies have furthermore indicated that in severe forms of AD, the circadian rhythm is phase delayed. Aged AD-prone mice also display perturbed sleep-wake patterns, which can be corrected by immunization against amyloid beta or by an orexin antagonist. Further research will help uncover the primary pathogenic contribution of the circadian system, and its independent contribution from perturbed sleep, in conditions like AD. Notably, evidence suggests that interventions that increase daytime light exposure and include melatonin supplementation are able to ameliorate symptoms of AD, presumably by counteracting disrupted circadian rhythms.

While the relation between shift work and depression has not been extensively studied, disruption of sleep and circadian rhythms and the pathogenesis of depression are intimately interlocked. Clock genes have also been implicated in depression and mood both in animal and human studies. Polymorphisms of genes that regulate sleep and circadian rhythms—for instance, a long gene variant of PER3—have also been linked to bipolar disorder and schizophrenia, while CRY2 and CLOCK gene polymorphisms are associated with seasonal affective disorder, a type of depression arising in the fall and winter months when the levels of sunlight are lowest. Bipolar disorder is furthermore often triggered by circadian disruptions or curtailed sleep. Both bipolar disorder and schizophrenia have been linked to various forms of circadian disruption following disease onset, and a critical component of disease treatment often involves normalizing sleep and sleep-wake rhythms.

Sleep deprivation by itself is known to reduce alertness, impair decision-making, and increase risk for accidents—after 18–24 h of continuous wakefulness, several skills exhibit the same degree of decline as following mild alcohol intoxication. However, cognitive abilities may suffer even further when sleep restriction is combined with circadian misalignment as in shift work. In one study, participants were subjected to ~33-h long days in parallel with reduced sleep (equivalent to 5.6 h sleep in a 24-h period), yielding a forced desynchrony protocol coupled with sleep loss. When subjects were tested at the nadir of their circadian period, the subjects’ reaction speed dropped almost by an order of magnitude compared with controls. In another study, researchers noted almost a 36% greater incidence of serious medical errors in resident interns who regularly worked 24-h or longer shifts compared with those who were randomly assigned to work up to 16-h long shifts. Furthermore, errors that resulted in patient death were three times more likely to occur in residents working extended hours compared with those who only worked up to 16-h long shifts.

Circadian Regulation of Gastrointestinal Homeostasis and the Microbiota

Physiologic aspects of the gastrointestinal (GI) tract exhibit day-night variations that anticipate and prepare for food intake and digestion during the active period. Gastric emptying, as well as colonic motility, are considerably greater during the active phase, as the phasic motor program supporting movement of digested material along the intestine is approximately twice as fast during the day compared with night. Bile acid secretion also exhibits circadian rhythmicity in the intestine, as does absorption and the expression of many nutrient uptake transporters in the intestinal wall, including the main glucose transporter protein SGLT1. The permeability of the intestinal wall also varies throughout the sleep-wake cycle, and mice exposed to chronic sleep fragmentation exhibit increased intestinal permeability, which may enable inflammatory molecules from bacteria to reach the systemic circulation.

The composition and function of the microbe population living in the intestine (i.e., the gut microbiota) also display circadian rhythmicity, orchestrated by both host circadian clock gene expression and food intake rhythms. Accordingly, circadian disruption, either by environmental or genetic means, perturbs these microbial rhythms, disrupting both bacterial levels and the metabolic functions of the gut microbiota. For example, alterations in the expression and functions of the gut microbiota have been noted in humans exposed to acute jetlag, and evidence suggests that curtailing sleep, which often accompanies shift work and jetlag, can alter the gut microbiota. By increasing local and systemic inflammation, circadian disruption of the gut microbiota may be causally involved in the increased risk of inflammatory bowel disease (Crohn’s disease and ulcerative colitis) and colon cancer in shift workers. Gender-specific differences have also been reported, as female mice display more pronounced microbial rhythms. Interestingly, the gut microbiome has also been shown to influence the rhythms of host tissues, such as the intestine and liver, suggesting that a bidirectional relationship exists between tissues that regulate metabolic processes and the gut microbiome across the sleep-wake cycle. These findings may furthermore have clinical implications, given that the gut microbiome may both directly (in the gut lumen) and indirectly (through host-microbiota interactions) impact pharmacokinetic and pharmacodynamic properties of therapeutic drugs across the 24-h day-night cycle.

Cardiovascular Health and the Circadian Clock

An early epidemiologic observation was a greater incidence of myocardial infarction in the morning hours, with the lowest risk during the period preceding sleep. Other cardiovascular outcomes such as sudden cardiac death and syncope also exhibit a daily peak in the morning. Blood pressure (BP) typically peaks around 21:00 h and decreases later during sleep, partially due to a circadian nighttime dip of around 3–6 mmHg in systolic BP and 2–3 mmHg in diastolic BP. A dip in blood pressure of either less than 10% or greater than 20% during normal sleep has been associated with worse cardiovascular prognosis. Heart rate also typically decreases during sleep, while mistimed sleep leads to higher heart rate during their sleep time. Thus, a combination of reasons—which may also involve altered glucocorticoid levels and increased platelet aggregation—may contribute to a greater risk of cardiovascular disease in the morning. Subsequent epidemiologic studies also have demonstrated that shift work increases the risk of dyslipidemia and hypertension, as well as the risk of coronary heart disease, including myocardial infarction. These findings are in line with interventional findings in which circadian misalignment has been induced either by inverting the sleep-wake cycle or by imposing 28-h days on healthy human subjects. These studies have found that circadian misalignment elevates 24-h blood pressure, particularly during sleep. These changes may be causally related to how the autonomic system is regulated during sleep, as evidenced by reduced vagal cardiac control when the sleep-wake cycle is inverted.

Circadian Disruption and Cancer

In 2007, the International Agency for Research on Cancer declared that shift work that involves circadian disruption is likely carcinogenic to humans. While evidence for an association between shift work and general cancer incidence is mixed, accruing evidence supports a link between shift work and increased risk of developing colon and breast cancer, as well as having a poorer cancer prognosis. Telomere shortening, a phenomenon in aging that destabilizes the genome, has also been observed in shift workers as well as in individuals suffering from short sleep. This may reduce the ability of damaged or senescent cells to undergo apoptosis, and instead lead to uninhibited cell growth and cancer. An indirect role for the circadian clock has also come from retrospective studies on how cancer risk is related to food timing and duration of the nighttime fast in humans. These studies indicate that by portioning food intake to a restricted period of the day, circadian processes are optimized in a way that confers reduced risk of carcinogenic cell damage. Studies of recurring fasting have also been shown to lower the risk as well as to delay the onset of cancer.

Experimental genetic evidence has also implicated clock disruption as a factor in tumorigenesis. Genetic loss of Per2 or Bmal1 has been shown to promote lung tumorigenesis, while studies in Per2 mutant mice have also revealed increased radiation-induced lymphoma associated with dysregulation of the cell cycle. However, disruption of the Cry gene in mice has also been implicated in tumor protection due to increased susceptibility to cell death. Thus, while both epidemiologic and experimental evidence suggests a link between circadian disruption and cancer, a full understanding of the role of circadian systems in tumorigenesis remains an area for investigation.

Circadian Regulation of the Immune System

Circadian misalignment and sleep restriction both alter population levels of immune cells and decrease the ability of immune cells to produce reactive radicals. Chronic circadian disruption may thereby impair the immune system’s ability to conduct immunosurveillance at the proper time of day and reduce the ability to mount an appropriate response upon exposure of pathogens during the recovery/sleep phase, when the immune system is typically more active. Instead, circadian misalignment increases a range of clinically used inflammatory markers (e.g., C-reactive protein, tumor necrosis factor α, and interleukin 6), and such changes have been noted even when the sleep-wake cycle is only prolonged to a slightly longer than normal 24.6-h day. While similar effects are also observed following acute total sleep deprivation or recurrent partial sleep restriction, circadian misalignment has been found to promote an even more pronounced elevation of such markers. Genetic clock disruption in peritoneal macrophages has also revealed clock control of Toll-like receptor 9, which is responsible for identifying molecules from foreign pathogens. Clock knockout mice also have reduced T cell antigen response, and mice immunized during the day had a stronger T cell response than mice immunized at night, supporting regulation of the immune system by the clock.

Aging and the Circadian Clock

Instability in the clock system is an often overlooked hallmark of aging. Aging is associated with a decline in the robustness of intrinsic rhythmic processes at the behavioral, physiologic, and molecular levels in both human and animal models. At the behavioral level, aging leads to reduced and fragmented sleep, dampened locomotor activity and feeding rhythms, and a reduced ability to entrain to light, as old rodents are 20 times less sensitive to the entraining effects of light relative to young animals. Even middle-aged individuals exposed to jetlag also exhibit more symptoms of circadian misalignment, such as more time awake and reduced alertness, compared with young individuals. On a physiologic level, some of the hallmarks of aging are a reduction in amplitude (e.g., flattening of circadian pattern) and a phase-advance (e.g., a shift in the timing of the peak or nadir) in rhythms of the endocrine and neuroendocrine systems, including sleep onset and offset. For example, cortisol, DHEA, and melatonin all have dampened rhythms and are phase-advanced in aging; the combination of such changes may, for instance, contribute to more fragmented sleep and lower levels of restorative slow-wave sleep in aged individuals. Aging also leads to alterations in peptide expression in the SCN (VIP and AVP) and reduced amplitude of rhythms of SCN electrical activity. Further, while the SCN-dependent body temperature rhythm—a generally accepted marker for the integrity of circadian rhythms—peaks in the evening and is lowest in the early morning in young individuals, aged healthy subjects display a phase advance and decrease in circadian amplitude in body temperature rhythms. Indeed, evidence suggests that internal desynchrony between core body temperature rhythms and the sleep-wake cycle may contribute to age-associated circadian alterations. On a molecular level, aging is associated with decreased expression and altered diurnal profiles of several of the core clock genes, including Clock and Bmal1, within both SCN and peripheral tissues such as heart and liver. Interestingly, the acute induction of Per1 in response to light was markedly reduced in the SCN of aged mice compared with young mice, potentially contributing to their delayed response to light entrainment. Mice lacking Bmal1 die prematurely compared with control mice, consistent with premature accumulation of reactive oxygen species. These mice have an accelerated onset of numerous age-related pathologies, including cataracts, sarcopenia, reduced organ size, and decreased hair growth. Instead, deficiency of Cryptochrome, a repressor of the internal clock repressor, has been associated with alterations in liver regeneration, while BMAL1 and PER2 may be important for proper neurogenesis in the hippocampus, a brain region in which adult mammals normally exhibit continuous cell division. Altogether, this suggests that the highly conserved circadian clock is important for regulating a wide range of homeostatic processes, including cell-cycle pathways, that when properly phased to each other promote organismal fitness.

Measurements of altered circadian rhythms with age may serve as a useful biomarker for aging. An intriguing question is whether the decline in amplitude of rhythms correlates with a decline in function, and importantly whether restoration of these rhythms with age, through either behavioral or pharmacologic intervention, would delay the aging process. Of note, transplantation of the SCN from a young rat into an old rat “rescued” the rhythms of both locomotor activity and corticotropin hormone (CRH), suggesting that the SCN is an important target for age-related changes in clocks. Treatments targeting the SCN may therefore ameliorate some of the deterioration in aged individuals.

Chronopharmacology, the study of how the timing of drug administration may impact its effectiveness, is a rapidly emerging field. Since physiologic processes vary across the day, the timing of administration of medication may help optimize patient care. For example, since endogenous cholesterol synthesis is rhythmic in liver and peaks during the early morning hours, administration of statins (HMG-CoA reductase inhibitors) in the evening prior to bedtime has proven to be more effective than daytime administration at reducing low-density lipoprotein cholesterol (LDL-C) levels because the highest concentration of the medications coincides with the peak in the rhythmic endogenous cholesterol production. Given that blood pressure exhibits a 24-h rhythm—being lowest during sleep—angiotensin-converting enzyme (ACE) inhibitors have been shown to be most effective at night to normalize the blood pressure rhythms, restoring the nighttime dip in blood pressure. Administration of cancer treatments according to circadian rhythms has also been shown to increase chemotherapy effectiveness while decreasing toxicity in a wide range of drugs. For example, 5-FU works best to treat colorectal cancer when administered at night, a time when the cancerous cells are more vulnerable while normal cells are quiescent and therefore less sensitive. Doxorubicin administration early in the morning to treat ovarian cancer has also been shown to be less toxic, as white blood cells recover faster than if the drug is given in the evening. Finally, the more severe morning symptoms of rheumatoid arthritis are linked to increased inflammation during the evening; therefore, prevention of the night-time upregulation of the immune/inflammatory reaction is more effective when glucocorticoids are administered with a night-time release formulation.

Recognition of circadian rhythms is also critical for diagnoses and treatment of endocrine disorders. The diagnosis of Cushing’s syndrome, which is characterized by hypercortisolemia, might be missed if the patient’s cortisol levels were measured in the morning, as endogenous cortisol production is typically highest during morning and lowest at night; therefore, clinical diagnosis requires cortisol to be measured in the late evening when the levels of this hormone should typically be low. On the other hand, adrenal insufficiency is diagnosed by measuring cortisol in the morning when at its physiologic peak, and glucocorticoid therapy for these patients aims to mimic the endogenous rhythms of cortisol, as short-acting synthetic glucocorticoids are usually given several times a day in tapering doses, such that the largest amount is taken in the morning and the smallest in the evening. Diabetes is another endocrine disorder intimately tied to circadian rhythms. Oral glucose tolerance has repeatedly been shown to be impaired in the afternoon and evening compared to the morning, likely due to a combination of a circadian regulation of insulin sensitivity within peripheral tissues and reduced insulin secretion during the night. Similarly, due to a surge in hormone levels in the morning, diabetes patients may suffer from the dawn phenomenon (or dawn effect), an abnormally high morning increase in blood glucose due to impaired response in insulin secretion. A related phenomenon that can be tied to evening timing of insulin doses is the “rebound” or Somogyi effect. In this scenario, the initially noted clinical sign, in the form of elevated glucose levels, may be noted in the morning. However, the underlying cause is hypoglycemia occurring during the night, which produces a counterregulatory hormonal response that subsequently results in morning hyperglycemia. As patients with type 2 diabetes often lack these daily cycles of insulin secretion and glucose tolerance, this further highlights that time of day is an important consideration for the diagnosis and treatment of metabolic disorders such as type 2 diabetes.

As our knowledge of the complexity of the interaction between circadian and physiologic processes deepens, further advances to rationally develop new strategies for treatments of disorders affected by circadian misalignment are essential. For example, novel compounds have begun to emerge from unbiased drug discovery screens that impact circadian clock components, either shortening or lengthening the period, including CRY stabilizers or various inhibitors of CKIδ, CKIε, and GSK-3. Pharmacologic control of the circadian cycle may be useful in the treatment of circadian disorders and metabolic disturbances with a circadian component. Understanding how the circadian clock controls biological functions will shed new light onto the pathogenesis of metabolic disorders with a circadian component, such as type 2 diabetes and metabolic syndrome, and will yield insight into how timing of drug delivery will impact patient care.

ACKNOWLEDGMENT

The authors would like to thank Billie Marcheva for her help with the figures and tables.