The Function of Sleep in Animals

Sleep has been observed in pretty much all animals that you can imagine. Jellyfish have been observed to exhibit some type of sleep like behaviour, as well as the worm or the nemotode,. So neither of these organisms even have a nervous system. But mammals do sleep vastly different amounts. bats, for instance, sleep 18 to 20 hours per day, while others, such as a giraffe, only sleep three or four hours a day. The fact that they had to evolve a special mechanism of uni hemispheric sleep seems to indicate ththere's some important function that served by sleep that they needed to maintain.