[grhino]

One function of sleep may be to clean up waste accumulated while awake. To run the cleaning process while awake may require more active energy consumption and was a way of conserving total energy in our evolutionary past when activity wouldn't be very productive anyways.

http://www.nih.gov/news/health/oct2013/ninds-17.htm

[jtchang]

So basically we run garbage collection for 8 hours every night.