They make a quiet sound in response, pleased and relieved to hear that Sans is so close by. They don't often wake up in the middle of the night, but knowing there's someone around they can find easily when they do is a comforting thought.

Frisk is starting to drift off when Sans speaks up again, making them mumble and blink their eyes open again. They shift a little, waiting to either be carried in or let down to get into bed themself. (Or maybe Sans will float them in, they like it when he does that.) "...mm-hmm."