Pythons, bats and (giant) armadillos are among the longest sleepers at over 18 hours a day. Human babies need 16 hours, and most of us (probably) feel we need around eight hours sleep to function well.
Professor Jerry Seigel from the University of California, Los Angeles, (conducted) a study of the sleep times of a broad range of (animals) and found that they vary widely. Some, like (migrating) birds, can (survive) long periods without sleeping at all. He believes that shows sleep evolved to conserve (energy):
Jerry Seigel: 'It's animals that are needlessly (active) that will not survive, but animals that are most efficient and use their waking time to do (vital) functions, and are otherwise asleep that will survive.'
Sleep helps make best use of limited (resources). In humans, when we're awake, our brain accounts for 20% of the (energy) we use when just sitting around. Sleeping also makes us less likely to get injured and less likely to be detected by predators.
Jon Stewart, BBC News