1) Hunger/happiness stats don't change during sleep
2) A tamagotchi won't leave if it is simply unhappy
3) It will be fine tomorrow as long as you wake up in time to play with it (or, set the clock so it wakes up later)
If you want to get Mamechi/Mimichi, change the time to when it would normally be awake, play with it, then return it to the normal time. If you don't care about what character you get, just leave it be and check it in the morning.
