Comment by llamasushi

Piggybacking off this, for a more general reason for sleep: "My definition would be as follows: sleep evolved as a species-specific response to a 24-hour world. During sleep – a period of physical inactivity – individuals avoid movement within an environment to which they are poorly adapted, but then use this time to undertake essential housekeeping functions demanded by their physiology."

From Life Time by Russell Foster. Still one of the most lucid and well-written books on sleep I've ever read.