Cats generally have a very high metabolic rate. Cats do sleep a lot of the time, though I would say 18 hours out of every 24 is a little higher than normal. 14-16 hours is far more typical. But with such a naturally high metabolic rate, they're lucky enough to be able to burn off calories even whilst fast asleep. Lucky old cats! The only time you will ever see an obviously overweight cat will be for one of only three reasons. Either a defective endocrine system (i.e. underactive thyroid gland), age, or an uneducated owner who simply feeds it far too much.