Metabolism

Cats conserve energy by sleeping more than most animals, especially as they grow older. Daily durations of sleep vary, usually 12–16 hours, with 13–14 being the average. Some cats can sleep as much as 20 hours in a 24-hour period. The term cat nap refers to the cat's ability to fall asleep (lightly) for a brief period and has entered the English lexicon – someone who nods off for a few minutes is said to be "taking a cat nap".

Source : http://en.wikipedia.org/wiki/Cat#Anatomy_and_morphology