Sleep is an altered state of the nervous system, a state without consciousness. All animals with a nervous system seem to need sleep in order to rest and recover. It is a dynamic process that seems to be crucial for an organism's well-being.

Unlike hibernation, coma, or inebriation, sleep is a unique state with its own distinct characteristics. The criteria for identifying sleep involve observable changes in behavior and physiological patterns. These criteria often include a reduction in physical activity, altered responsiveness to stimuli, and characteristic changes in brain wave activity, such as the transition between REM (Rapid Eye Movement) and non-REM sleep stages.

The mystery of sleep revolves around three primary theories. The first suggests that sleep allows restoration and detoxification in the body. The second proposes that it enhances cognitive performance by reinforcing or pruning synaptic connections. The third theory focuses on energy conservation, with animals adapting sleep duration based on their ecological niche and foraging needs. Obviously, these theories are not mutually exclusive.

Sleep durations vary widely among species. For example, the little brown bat may snooze for 20 hours a day, the African bush elephant typically sleeps just two hours, the giraffe averages around 1.9 hours, and humans generally require 7-9 hours of sleep.