A recent article in Newsday states they believe flies sleep, react to caffiene by sleeping less, and make up for restless periods by sleeping more the following day. They also talk about how mammals die when deprived of REM:
In humans (or our dogs), an EEG (electroencephalogram) can trace the distinctive electrical waves of a sleeping brain. But scientists still don’t know exactly what sleep is for. They do know sleep is essential for life in mammals and birds, all of which sleep and apparently dream. Reptiles sleep, too, though scientists aren’t sure if they dream. Sleep is so important that an animal deprived of it for long enough will eventually die. In fact, animals will also die when deprived of just the dreaming part of sleep, also known as REM (rapid eye movement) sleep.
Until recently they were unsure if reptiles even slept, but isn’t it hard to imagine going without a good night sleep and the occassional siesta?
Sleep is better than sliced bread, as noted in the article:
Sleep clearly recharges the body, but it may exist mainly for the benefit of the brain and nervous system. Scientists say that while we sleep, the brain does its daily cleaning and organizing, erasing inessential memories and consolidating and storing others. After a good night’s sleep, we wake up with a relatively clean slate.