[jacques_chester]

Sleep is also implicated in muscle repair, aceytlcholine turnover (related to memory formation and muscle power output) and ... well ... pretty much everything.

If sleep wasn't performing useful functions, it would have been selected against ever since higher animals evolved.

[astrodust]

Unless, of course, stumbling about in the dark and unable to see while bored out of your skull because you can't sleep posed some sort of risk.

[bpodgursky]

Owls and lots of cats have no trouble seeing at night, and still sleep for half the day. I can't think of any organism with high-level cognitive ability that doesn't sleep.