Cats sleep 15 to 20 hours per day because they are typically nocturnal, they conserve energy, and they are light sleepers. The amount of sleep a cat gets is often relative to how it spends its time awake.
The hours between dusk and dawn are typically prime time for cat activity. Even though cats are domesticated, they are nocturnal predators as a species, and it is natural for them to express their wild streak at night. Long periods of light sleep help cats conserve energy, while their light dozing keeps them in a position to pounce at any time. Pet cats often adapt to their owners' schedules and can adapt their sleeping time around prime play and activity time.