Why Do We Sleep?

Many scientists believe that sleep must serve some crucial purpose, since sleeping animals can't do useful things like search for food and may be easier targets for hungry predators. Memory processing has been held up as the critical task that might have made sleep worth the risk.

"The literature's all over the place in terms of what stage of sleep and what type of memory" is supposedly affected, says Jerry Siegel, a maverick professor of psychiatry at the University of California at Los Angeles. "The evidence isn't converging. It's contradictory."

Read full story in Slate.