http://www.obee.ucla.edu/...Kavanau/OriginEvolutionSleep.pdf
Biomechanically, sleep is used to restore "synaptic efficacy," or simply put, it keeps you from overstraining the nervous system and having a nervous breakdown. I'm not sure that there's necessarily a specific and direct survival advantage to sleep except that it seems to be a necessary evil of a developed nervous system, and having said system is itself a survival advantage great enough to outweigh the costs of sleep.
[This message has been edited by ::, 12-09-2003]