Evolution of Sleeping

Are there any theories as to why sleep evolved? Is it just an inevitable consequence due to different selective pressures, or did it evolve specifically? It seems kind of absurd to remain completely unaware of your surroundings for 8-10 hours a day. I haven't slept in 3 days because of finals, so this may or may not be the good question I think it is.

http://www.obee.ucla.edu/...Kavanau/OriginEvolutionSleep.pdfBiomechanically, 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]