Definition and Function of Sleep
Sleep may be defined as an episodic and reversible behavioural state characterized by decreased responsiveness to external and internal stimuli. Sleep should not be considered a passive process ie. what is left over after the body shuts down. Rather sleep is a complex and dynamic state, with the body continuing to experience a multitude of behavioural, physiological and cognitive (mental) processes.
Virtually all animals sleep in one form or another. Humans spend about one third of their lives sleeping. These facts alone suggest that sleep serves a critical biological function or need. Surprisingly, however, the function of sleep has not been fully elucidated, although many theories have been proposed. The simplest theory proposes that sleep serves to conserve energy for the organism, since organisms are relatively inactive during the state of sleep. In an extension of this theory some propose that sleep does more than merely conserve energy, but also serves a restorative function for the organism, be it physical or mental. Sleep has also been proposed to serve a function in temperature control and immune function. Sleep may also serve to consolidate memory, in particular REM sleep. Finally it has been proposed that sleep serves an adaptive function, serving to prevent an organism from wandering around when doing so may be dangerous to its survival (such as at night for diurnal animals).
