From Creatures Wikia
Sleep is necessary to process instincts in the brain. All creatures sleep - if they do not, bad things happen, like forgetting that it is a good idea to sleep . . . which makes things worse and causes them to forget a lot of other things, like food being a good idea when you are hungry*, often leading to death. Compare with Rest.
Did you know? Death occurs in real life for rats after about 14 days of sleep deprivation.