My understanding is that one's basal metabolic rate is for when you are inactive. Of course, when you are asleep, you are not moving. But I would guess there are differences in how the body metabolizes when you are awake but deliberately lying prone, and when you are asleep.
Near as I can figure, my BMR is roughly 1700, not including exercise or daily activity. Which means, that if I sleep for 8 hours, I should burn about 560 calories, give or take.
Is it reasonable to assume the same BMR for sleeping hours, or should one adjust up or down?