[Cotterzz]

It has a lot to do with diet and metabolism. Cat's sleep 2/3 of the time, have a very high metabolism and spend a very short amount of time eating or catching prey. (compared to say, Cows who are chewing for as long as humans are sleeping) So it's likely to conserve energy in situations where there is a limited food supply or constraints on what and when you can eat. Then there's animals that hibernate, for similar reasons, over winter. But that's only one aspect of sleep, there's also the file indexing and disk defragging that goes on when you're not running any applications.