Sleepy Time for Mammals

The funny thing about sleep is that we all need it. It is very important and it still confuses scientists as to why. There are many different mammals on the planet and almost as many different ways to sleep, and sleep requirements. Why is it that some of the smallest mammals need the most sleep and some of the biggest need the least? Why do mammals hibernate, and some do not? How can some animals that need to breathe air, sleep underwater?