I've not had that problem before but two things come to mind. Is there an alarm clock in the room or does the sim have an early work shift?
I know my school age kids wake up at 7, an hour before the school bus arrives, as do sims who have jobs that start early in the morning.
Or maybe your sim is a light sleeper and wild horses or deer are running through the lot at that time?
If it's neither of those, have you tried buying a different bed, or sleeping in a different room?
If you have NRaas Master Controller, you could always reset the sim, the lot, or the town, to see if that helps.
I did a little research about the issue and also found this information:
https://sims3.crinrict.com/en/2011/06/bug-sim-wakes-up-every-morning-at-4-am.htmlLet me know if any of that helps and if not, we can research it some more.