He has sex with me while I'm sleeping..

Okay so this might be extremely weird or just a common kink, I really have no idea, that's why I'm asking. Well my boyfriend has a hard time falling asleep so I always fall asleep before him, and one day I woke up to him touching me and undressing me. I was startled, and so I asked him what was going on and we just had regular sex. So he continued to do this sparingly, and one night I just decided to pretend I was still asleep. He continued on and seemed very turned on by the whole thing. Now, it has become more regular and I notice it puts him right to sleep. We still have sex while I'm awake obviously, but I'm just wondering if anyone has ever heard of this fetish or do you think I should be worried?