Evolutionary Theory of Sleep:

Evolutionary theory, also known as the adaptive theory of sleep, suggests that periods of activity and inactivity evolved as a means of conserving energy. According to this theory, all species have adapted to sleep during periods of time when wakefulness would be the most hazardous. Animals that have few natural predators, such as bears and lions, often sleep between 12 to 15 hours each day. On the other hand, animals that have many natural predators have only short periods of sleep, usually getting no more than 4 or 5 hours of sleep each day.

Van Wagner, Kendra (©2009 ). Theories of Sleep. Retrieved January 16, 2009, from about.com Web site: http://psychology.about.com/od/statesofconsciousness/p/TheoriesofSleep.htm