Cats - even big cats - sleep most of the time.  Bored cats sleep as
much as twenty hours a day.

There is much speculation about this, but the explanation that makes
the most sense to me is the observation that cats can hunt efficiently
only around dawn and dusk - these are the periods when activity (in
the wild) is most likely to result in calorie intake - activity at
other times consumes calories without producing any, and therefore is
wasted.  Food is the limiting factor: there is seldom more food
available than just enough to support the predators without
exterminating the prey - if the predators waste activity, they also
waste food, which results in either fewer cats in the area, or if they
eat all the prey, the death of all the cats in the area.

Domestic cats with unlimited food available, and something to keep
them interested don't sleep all that much: mine are awake and at least
alert for about 10-12 hours a day.

T.E.D. (tdavis@gearbox.maem.umr.edu - e-mail must contain "T.E.D." or my .sig in the body)