Cats sleep on people for a variety of reasons:

1. Comfort and Warmth: Humans are warm, so your cat might find snuggling up against you comfortable and cozy. This is especially true during colder months when they're seeking out heat sources.
2. Bonding: Cats form strong bonds with their owners. By sleeping on you, they're showing a level of trust and affection. It's a way for them to feel close to you and reinforce their bond with you.
3. Security: Cats are vulnerable when they sleep. Choosing to sleep on you indicates that they trust you and feel secure in your presence. This behavior can be traced back to their kittenhood, when they would sleep piled on their siblings and mother for safety.
4. Scent Marking: Cats have scent glands throughout their bodies. When they sleep on you, they transfer their scent onto you, marking you as part of their territory. This is a compliment in the feline world, signifying that they consider you part of their family.
5. Attention: Cats know that if they're right on top of you, they won't be ignored. This position makes it easy for them to get your attention (and perhaps some petting) as soon as you wake up.

As with all behaviors, individual cats may have their own specific reasons or habits. Always remember to respect your cat's boundaries, even while enjoying these tender moments.