so I have experience this, and many are talking about this, when you are in bed in the night, Face ID just stop working? I think I may have found the cause for this. a lot of us who are experiencing this wear glasses. when we are in bed, we take off the glasses, and thus we hold the phone a lot closer to our face, even without us noticing it. if you just move your phone a little further away from your face, Face ID starts working again. just give it a try.