However, studies on humans sleeping in constant environments without external time cues show that sleep episodes tend to be shorter following long periods of wakefulness. Studies on the effects of exercise on subsequent sleep also do not appear to support the body restoration hypothesis. According to this theory, one would predict increased sleep following elevated catabolism produced by physical exertion, but studies show no effect on post-exercise sleep. A second theory is that sleep serves an energy conservation function. According to this hypothesis, the body needs sleep in order to offset the high energy costs of endothermy.
Sleep reduces the metabolic rate and body temperature in endothermic animals.
When animals fall asleep their metabolic rates decrease by approximately ten percent. Heat is dissipated from the body through peripheral vasodilation, leading to a one to two degree reduction in body temperature. Data in favor of this hypothesis rest on the similarities between hibernation and sleep. Infections and the ensuing fever often induce sleep because cytokines, chemicals produced by the immune system to fight infection, have sleep-inducing properties. Furthermore, the current view of cytokines is that they are predominantly inflammatory mediators.