Sleep is a natural state of rest for members of the animal kingdom. Scientists have observed the sleep of mammals, birds, reptiles, fish, and amphibians. Although we are still not entirely sure of how sleep works, nor are we convinced that we understand all of the functions of sleep, scientists have become convinced that sleep is necessary to survival.

There are a number of theories about why we sleep and what happens in our bodies and brains during sleep cycles. However, there is not currently one predominant theory. It is quite possible that there is a kernel of truth in each of these theories; that they will all work together eventually to inform a more complete understanding of human sleep.

Most scientists agree that one of the major purposes of sleep is to restore and heal the body. It has been observed that hormone and immune functions change during specific stages of the sleep cycle. Furthermore, some studies have shown that sleep deprivation can lead to deficiencies in the immune system. Although it is believed by some that important growth can take place during sleep, there have been no studies to show that the lack of sleep can halt or stunt growth.

It has also been hypothesized that sleep offers important restoration to the brain. It is possible that neurons are restored, that brain proteins and certain hormones are produced. Some scientists believe that sleep is particularly important to the brains in young humans. Sleep deprivation in early life has led to decreased brain mass, permanent sleeping trouble, and behavioral problems. Although many scientists feel that the main function of sleep is not to restore or improve the capabilities of our memory, it has been observed that people have an easier time memorizing information if they have had adequate sleep than if they are sleep deprived.

A completely alternate theory to those described above is the “Preservation and Protection” theory of sleep. This theory asserts that human beings do not require the full 24 hour period within each day to satisfy basic needs such as collecting necessary food and supplies, eating, and reproducing. As not all 24 hours are required, sleep offers a time of rest when humans are not out in the elements, and therefore exposed to threats. Just as the caveman is less likely to be pounced upon by a jaguar while he is tucked inside of his sleeping space, the modern man is less likely to be hit by a bus while sleeping in his apartment. This theory, of course, does not postulate what happens inside of our bodies and brains while we sleep.