Remember that sleep evolved very early on. Arthropods (insects, arachnids, crustaceans, etc.) sleep. Worms experience a sleep-like state. It's even been shown t

Remember that sleep evolved very early on. Arthropods (insects, arachnids, crustaceans, etc.) sleep. Worms experience a sleep-like state. It's even been shown that cyanobacteria experience circadian rhythms. Fruit flies whose sleep patterns were disrupted were slower to learn new things and faster to forget the things they learned. I'm not sure there is a single purpose for sleep, but memory is certainly a critical factor.