Every living thing has an internal clock. Some people have overridden their natural tendencies, but animals remain true to their natural inclinations (unless people intervene). Domestic cats are still fairly closely related to their early feline ancestors. Unlike dogs (who have gradually modified as they have evolved with humans) in many ways, our cats are unchanged.
All felines are crepuscular (active at dawn and dusk) and like to ramp up activity in the early morning and at dusk. This tendency is thought to be an adaption that makes cats most active when other dangerous predators (who are nocturnal) are not. This means that at other times of day, cats like to conserve energy and nap. Normal cats sleep a lot.
There are conditions that are not normal that cause cats to be hyperactive or even lethargic, so it is important to know your own cats. If you think that your cat is sleeping too much, it is smart to ask your vet.
Regardless of whether or not he sleeps a lot or a little, be sure that your cat is seeing your veterinarian regularly. Because they are not extremely evolved from their wild predecessors, cats are masters at hiding disease and they instinctively avoid flagging themselves as weak to avoid predation. All cats need to have a full veterinary exam at least once yearly, but if your cat sleeps a lot and always has, he is probably just being himself!
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