Both my boys, when they sleep with me on the bed, will curl up either on/below my feet, behind my knee joint, or at my waist. I don't think it's just because there's a crook there to snuggle into (in the case of my feet, it's definitely not that). My guess is that my body gives off marginally more heat at the joints.
OK, Matti is pestering the heck out of me. I'm going to go feed him, in case that's what he wants (even if not, it'll distract him for a while).