The function of sleep remains one of the mysteries of the ages, yet sleep is an evolutionarily preserved behavior manifest in diverse organisms including worms (1), flies (2), and all vertebrates thus far examined (3). Perhaps one of the most compelling arguments for a sleep function is its homeostatic control. In everyday language, this homeostasis usually refers to the tendency to sleep deeply after staying awake for a long period and, after sleeping over a long period, to stay awake.
