Sleep is forgetful. You never remember the exact time you fell asleep. There are always a few moments of la-la land, where you occasionally catch yourself and wake up but never remember all those times you didn't and you fell asleep. Sometimes when I lay awake all night and think that I only got a few hours of sleep, I'll find myself wondering if perhaps I was asleep much more. That maybe between my memories of laying awake and checking the clock at 2:00, 2:30, 3:30, and 4:00, I kept falling asleep but not remembering it. Then when I wake up at 8:00 after a few hours and feel very tired, I consider that I could have slept way more than I think I slept, and that I'm not that tired, and I'm just making myself feel tired by thinking that I slept so little. That train of thought usually keeps me awake until at least noon.