As well as yawning because they are tired (and cats do seem to have a very well developed sense of the importance of sleeping and napping), your cat or kitten may also yawn in order to show his affection. It is likely that this is his way of telling you that he feels safe and relaxed in your company and so does not need to be on the alert. Sleeping on your lap or on your bed is also a sign of their trust and affection.
Cats are expert sleepers. An adult cat will apparently spend most of their life sleeping, although much of the time they are in a light sleep (or "cat nap"). They only spend around a quarter of the day in a deep sleep. This behaviour originates for their need to conserve energy in the wild so that they do not need to hunt as often. As carnivores, their high protein meat diet also tends to make them sleepy. Kittens can sleep even more! This is partly because their growth hormone is released while they sleep.