Comforting physical contact
Most cats love going and getting into their human's bed in the middle of the night, just to sleep snuggled up with them. There is more than one reason for this behavior, and all of them are valid.
The first reason is pretty simple and practical, but no less true. Mittens likes to sleep in warm, cozy spaces, doesn't he? Well, you're warm and the bed is cozy.
But it's not just about the practical comfort: your cat is also looking for peace of mind. If they're nervous or there's something in the environment that is stressing them out, sleeping next to you will make them feel safe. Fortunately, this works both ways: if you're experiencing stress or worries, there's nothing more calming in life than the sound of your beloved little kitty purring next to you.