Normal sleep in adult humans is organized into cycles of non–rapid eye movement (NREM) sleep alternating with rapid eye movement (REM) sleep. NREM sleep is further divided into stages 1, 2, and 3, which are transitions as sleep deepens and which represent progressive slowing of the electroencephalogram (EEG). Stages 1, 2, and 3 of NREM sleep and REM sleep each can be identified by eye movements, characteristic EEG frequencies and patterns, and muscle tone. Stage 3 NREM sleep, also known as “slow wave sleep” (SWS), is associated with decreased heart rate, blood pressure, and sympathetic nervous system activity, as well as with decreased cerebral glucose utilization, growth hormone and prolactin release, and inhibition of cortisol secretion. Accordingly, SWS is the stage of sleep that is most highly metabolically and hormonally active. On the other hand, REM sleep is required for memory consolidation, and is the stage of sleep during which vivid dreaming occurs. Staging of sleep is important because different sleep stages may serve different biological functions. Each sleep cycle is approximately 90 to 120 minutes in duration. Stage 3 NREM sleep occurs predominately during the first half of the biological night (based on each individual’s circadian rhythm), whereas REM periods typically increase in length and occurrence as the biological night progresses (Fig. 11-1).
Hypnogram demonstrating sleep stages in a typical adult with normal sleep, starting from when the person fell asleep. Black represents time spent in non–rapid eye movement (NREM) sleep: stages 1 (N1), 2 (N2), and 3 (N3). Red represents rapid eye movement (REM) sleep.
Disordered sleep in humans may be insufficient (reduced in quantity), misaligned to the environment, disrupted, or a combination of these. Conceptualizing disordered sleep in this way is useful because insufficient, misaligned, and disrupted sleep differentially affects the structural organization (i.e., the architecture) of sleep. Hence, it is plausible that the effects of insufficient, misaligned, or disrupted sleep on biological systems, including metabolic and reproductive health, would differ depending on the sleep stages involved. Insufficient sleep requires, by definition, a reduction in total sleep, but stage 3 NREM sleep tends to be preserved at the expense of all other stages. The impact of circadian misalignment on sleep architecture appears to depend on whether there is a phase advance (sleeping earlier than usual) or a phase delay (sleeping later) in sleep. When sleep is phase-advanced, experimental studies show a decrease in REM and SWS and an increase in time awake. These same studies also show that there is increased REM sleep, decreased stage 2 sleep, preserved SWS, and increased time awake when sleep is phase delayed. With disrupted sleep, sleep architecture becomes completely disorganized. Disrupted sleep arising from obstructive sleep apnea is associated with frequent awakenings, fragmented sleep, and less REM sleep because the apneic episodes that result in partial awakenings tend to occur with ...