Research also suggests that adenosine builds up in our blood while we are awake and causes drowsiness, then gradually breaks down while we sleep. Furthermore, at least two distinct phases of sleep have been identified. Clearly, sleep is an active and dynamic state. Animal studies show that sleep is necessary for survival. For example, while rats normally live for two to three years, those deprived of REM sleep survive only about five weeks on average.
Rats deprived of all sleep stages live only about three weeks.
Sleep-deprived rats also develop low body temperatures and sores on their tails and paws. The sores may develop because the rats' immune systems become impaired. So the evidence suggests that there is a function of sleep other than to prevent sleepiness, just as there is a function of eating other than to prevent hunger. Various hypotheses that relate sleep to metabolic, immune, endocrine and brain functions have been proposed to answer this question. According to this theory, sleep allows neurons that are used in wakefulness to shut down and repair themselves.