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As outlined in Chapter 12, sleep occurs in distinct states classified broadly as non–rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. These states occur in a temporally organized fashion across the sleep period, with NREM dominating during the initial hours of sleep and REM during the later hours of sleep. REM sleep cycles occur in 90- to 120-min cycles, with a typical healthy adult experiencing 3 to 5 cycles during the major sleep bout. Specific alterations in autonomic regulation occur during REM sleep, which may at least partly explain the higher frequency of occurrence of myocardial infarction and ischemic stroke during early morning hours,1–3 when REM sleep predominates. This chapter presents additional information on alterations in cardiopulmonary function during sleep.


During NREM sleep, heart rate normally slows by 5% to 10%,4 and blood pressure drops by about 10%.4 These changes are most prominent in the deepest stage of NREM sleep,4 known as slow wave sleep (stage N3).5 The mechanism for this decline was elucidated by the seminal study of Somers et al.4 using microneurography to record sympathetic bursts in the peroneal nerve in healthy humans during wake and in the different stages of sleep (Fig. 100-1). These studies confirmed a fall in sympathetic activity during NREM sleep, which is responsible for the drop in blood pressure, cardiac output, and systemic vascular resistance.4 The fall in sympathetic activity is accompanied by an increase in parasympathetic (vagal nerve) activity that is believed to be responsible for the bradycardia of NREM sleep.6 Fundamental to NREM sleep is that the balance of parasympathetic/sympathetic activity is altered, with parasympathetic tone being dominant, thereby affecting heart rate variability during sleep. The high-frequency component of heart rate variability is said to reflect parasympathetic activity, while the low-frequency component is related to sympathetic activity.4 Thus, during NREM sleep compared with wakefulness and REM sleep, there is an increase in the high-frequency component and a marked reduction in the low-frequency component of heart rate variability.4

Figure 100-1

Alterations in recorded bursts of sympathetic nerve activity (SNA) and blood pressure (BP) in wakefulness, different stages of non–rapid eye movement sleep (NREM) (stages 2–4), and rapid eye movement (REM). With deepening of NREM sleep, there is progressive loss of sympathetic activity, which is virtually absent in slow-wave sleep (stage 4). Sympathetic activity returns in REM but is highly variable. (Reproduced with permission from Somers VK, Dyken ME, Mark AL, et al. Sympathetic-nerve activity during sleep in normal subjects. N Engl J Med. 1993;328(5):303–307.)

Changes in cardiovascular variables in REM sleep differ from NREM sleep. During REM sleep, there are surges of sympathetic activity such that heart rate and blood pressure return to, or may sometimes even ...

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