It has long been known that body temperature falls slightly during sleep; however, if sleep does not occur, there is still a drop in body temperature as part of the circadian (24-h) temperature pattern. This reduction in temperature is also independent of the 24-h recumbency–ambulatory cycle. During sleep, the decline in temperature occurs mainly during the NREM period, and the same is true of the heartbeat and respiration, both of which become slow and more regular in this period. Cerebral blood flow and oxygen consumption in muscle diminish during NREM sleep and increase during REM sleep. Also, cerebral blood flow and metabolism are markedly reduced across the entire brain during deep NREM sleep; however, during REM sleep, metabolism and blood flow are restored to the levels of the waking state (Madsen and Vorstrup). Presumably, as a result of increased blood flow, intracranial pressure rises during REM sleep.
Urine excretion decreases during sleep and the absolute quantity of sodium and potassium that is eliminated also decreases; however, urine specific gravity and osmolality increase, presumably because of increased antidiuretic hormone excretion, and reabsorption of water. Parasympathetic outflow is activated periodically in REM sleep; sympathetic activity is suppressed. It has also been recognized that there is a drop in blood pressure and heart rate during slow-wave sleep and the loss of this dip, for example, as a result of sleep apnea, has been associated with daytime hypertension and increased risk for cardiovascular events. As mentioned, in REM sleep, there is an increase in sympathetic tone. Breathing is more irregular, and heart rate and blood pressure fluctuate. Penile erections appear periodically, usually during REM periods.
A number of endocrine changes also have a regular relationship to the sleep–wake cycle. During the first 2 h of sleep, there is a surge of growth hormone secretion, mainly during slow-wave sleep. In men, there tends to be a single peak, whereas women have a multiple episodes of increased secretion. This feature persists through middle and late adult life and then disappears. The secretion of cortisol and particularly of thyroid-stimulating hormone peaks at the onset of sleep. High concentrations of cortisol are also characteristically found on awakening. Melatonin, elaborated by the pineal gland, is produced at night and ceases upon retinal stimulation by sunlight (see Chap. 26).
Prolactin secretion increases during the night in both men and women, the highest plasma concentrations being found soon after the onset of sleep. Secretion of prolactin is influenced by sleep stages. Circadian mechanisms and the stages of sleep alter testosterone secretion and are therefore disrupted by sleep disorders, especially in younger individuals. Also, an increased sleep-associated secretion of luteinizing hormone occurs in pubertal boys and girls.
Neurophysiology of Sleep and Dreaming
Hobson originally proposed that the basic oscillation of the sleep cycle is the result of reciprocal interaction of excitatory and inhibitory neurotransmitters. Single-cell recordings from the pontine reticular formation suggest that there are two interconnected neuronal populations whose levels of activity fluctuate periodically and reciprocally. During wakefulness, according to this conceptualization, the activity of monoaminergic (inhibitory) neurons is high; because of this inhibition, the activity of the cholinergic neurons is low. During NREM sleep, aminergic inhibition gradually declines and cholinergic excitation increases; REM sleep occurs when the shift is complete. It is likely that these monoaminergic neuronal circuits are modulated by input from orexin-secreting (also called hypocretin) neurons of the hypothalamus, but the details of this control system are not entirely known. Orexin, a peptide that assumes great importance in the pathophysiology of narcolepsy, is discussed further on. There is also emerging evidence from animal work that orexin is involved in autonomic homeostatic control as reviewed by Grimaldi and colleagues.