Why We Sleep: A Hypothesis for an Ultimate or Evolutionary Origin for Sleep and Other Physiological Rhythms

Although sleep is ubiquitous, its evolutionary purpose remains elusive. Though every species of animal, as well as many plants sleep, theories of its origin are purely physiological, e.g. to conserve energy, make repairs or to consolidate learning. An evolutionary reason for sleep would answer one of biology's fundamental unanswered questions. When environmental conditions change on a periodic basis (winter/summer, day/night) organisms must somehow confront the change or else be less able to compete in either niche. Seasonal adaptation includes the migration of birds, changes in honeybee physiology and winter abscission in plants. Diurnal adaptation must be more rapid, forcing changes in behavior in addition to physiology. Since organisms must exist in both environments, evolution has created a way to force a change in behavior, in effect creating "different" organisms (one awake, one asleep) adapted separately to two distinct niches. We sleep to allow evolving into two competing niches. The physiology of sleep forces a change to a different state for the second niche. The physiological needs for sleep are mechanisms that have evolved to achieve this goal.