Animal studies with rats have suggested that during sleep deprivation, energy expenditure increases while temperature and weight drop until body systems begin to fail, beginning with the endocrine and immune systems.y This and other studies have led to the proposal that sleep conserves energy loss through thermoregulation: When core body temperature decreases during sleep, heat loss to the environment is minimized. Similar studies have understandably not been performed in humans, but humans also show lowered body temperature during sleep. A different interpretation of temperature changes begins with the observation that sleep in general and deeper NREM sleep in particular are increased when the body is heated y ; therefore it is suggested that sleep functions to allow the shedding of excess accumulated heat.y This approach emphasizes not the decreased rate of heat loss in sleep due to decreased core temperature but the reduced rate of heat production due to slowed metabolism. Sleep may be the prime period for anabolic activity. In primates, growth hormone, whose effects can be manifest at all ages in humans, is primarily secreted during the periods of deepest SWS early in the night. y In several studies of growth in children with growth disorders, it was found that total sleep correlated with rate of growth, suggesting that disrupted sleep could inhibit growth.
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