CHAPTER 14 THE PHYSIOLOGY OF SLEEP
Sleep is an essential and appetitive behavior characterized by minimal movement; reduced responsiveness to stimuli; reversibility; and species-specific diurnal timing, duration, and preferred posture.1 The appetitive and essential nature of sleep is clearly evident in the human’s inability to maintain continuous wakefulness for more than 2 to 3 days. As a state of sleep need progressively increases with attempts at prolonged wakefulness, sleep begins to intrude into wakefulness as brief microsleeps occurring during ongoing behavior and as longer episodes of unintended sleep during periods of inactivity.2 The inability to completely deprive oneself of sleep after 2 to 3 days, in contrast to one’s ability to avoid food or fluids and thereby deprive oneself to death, demonstrates the compulsory nature of sleep. In fact, the compulsory nature of sleep accounts for much of the morbidity associated with sleep loss (e.g., car accidents).
Sleep in humans is recognized behaviorally by its recumbence and eye closure, but some mammals sleep with open eyes (e.g., cattle) or while standing (e.g., horse, elephant).1 The immobility of the sleep state is relative in that sleep walking and talking occur in some human sleep disorders and, among animals, some fish swim in place and mammals move about periodically. The two characteristics of arousability and rapid reversibility differentiate sleep from death, coma, and hibernation. Nonvisual sensory monitoring of both exogenous and endogenous stimuli continues during the sleep state. For example, a vital stimulus, hypoxemia, readily arouses even a severely sleep-deprived individual; similarly, a parent is easily aroused by the cry of his or her baby. In fact, sensory discrimination continues during sleep, inasmuch as a parent does not arouse to the cry of another baby, corrected for stimulus intensity differences. Average daily sleep time varies from 2 to 20 hours among mammals, typically being 8 hours for humans.3 The single best correlate of variation in sleep length among mammals appears to be metabolic rate.3 Whereas the major sleep period in humans typically occurs as a single bout during the dark hours, for some mammals sleep is linked to the daylight period and occurs in multiple bouts.
NATURE OF SLEEP
Because sleep would be disrupted if it were assessed behaviorally (e.g., testing arousal threshold), sleep scientists measure sleep electrophysiologically, which is less obtrusive and more precise.4 The simultaneous recording of the electroencephalogram (EEG), the electro-oculogram, and the electromyogram are the accepted standard measures of sleep and waking, a standardized procedure termed polysomnography. The polysomnogram correlates well with behavioral observations. But it also reveals further subtleties not apparent behaviorally or subjectively. Sleep is an active, complex, and highly organized process composed of two distinct brain states of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM).
Electrophysiology of NREM and REM sleep
The electro-oculogram of REM sleep is characterized by rapid conjugate eye movements (hence the name of this sleep state). The cortical EEG of REM sleep reverts to the low-voltage, mixed-frequency pattern of drowsy sleep. The second defining characteristic of REM sleep is its skeletal muscle atonia, which is reflected in the electromyogram achieving its lowest level of the night. The muscle atonia of REM sleep occurs through a process of postsynaptic inhibition of motor neurons at the dorsal horn of the spinal cord. Another important feature of REM sleep is its tonic and phasic components. The tonic components of REM sleep are the persistent muscle atonia and the desynchronized EEG. The phasic components are intermittent and include bursts of eye movements occurring against a background of electro-oculographic quiescence. Coupled with the eye movement bursts are muscle twitches, typically involving peripheral muscles. These twitches are superimposed on the tonic muscle atonia of REM and probably reflect sympathetic drive breaking through the postsynaptic inhibition (see the following discussion of the autonomic nervous system during sleep).
Physiological Function during Sleep
Autonomic Nervous System
The activity of the autonomic nervous system varies between the two sleep states (NREM and REM) and the wakefulness state.5 Parasympathetic activity increases during NREM sleep in relation to wakefulness. It remains relatively increased during both tonic and phasic REM sleep. Sympathetic activity remains constant during wakefulness and NREM sleep and is slightly reduced during tonic REM sleep. Consequently, parasympathetic activity predominates during sleep with the exception of phasic REM sleep. Sympathetic drive is dramatically increased during phasic REM sleep, and it predominates despite the increased parasympathetic activity of phasic REM sleep.
Respiratory System
Breathing patterns and the control of respiration are different in sleep and wakefulness.6 Minute ventilation is decreased from waking levels by 13% to 15% during NREM sleep. Two factors are responsible: First, the nonmetabolic drive to breathe in wakefulness is removed with the onset of NREM sleep; second, airflow resistance is enhanced, as a result of a reduction of upper airway dilator muscle tone that occurs in conjunction with the general reduction of skeletal muscle tone of sleep. During the tonic skeletal muscle atonia of REM sleep, airway resistance is further increased in comparison with that of NREM sleep, resulting in a twofold increase in relation to that of wakefulness. This heightened airway resistance, coupled with the autonomic nervous system sympathetic activation, particularly in phasic REM sleep, leads to irregular breathing patterns and even respiratory pauses during REM sleep.
