I'm having the same issue. This one Sim always wakes up at around 8am for some reason.
He doesn't have a new bed (He moved into his parent's old room with their old stuff. They never had this issue).
He has no sleep related traits.
He's a singer and doesn't have a regular job schedule so it's definitely not related to his job.
He doesn't have an alarm clock, tv, radio, or anything to disturb his sleep in that room. I also have his room locked so no one comes in and out.
He's been to bed in so many different situations (very sleepy, not too sleepy, low needs, high needs, early, late, high mood, low mood, etc) and the same thing still happens.
I mean, it's fine. I just let him sleep again as soon as he wakes up. But sometimes when I'm tending to other Sims, I forget about him and he just ends up doing his own thing. It's annoying when he has some tasks to do that day because obviously his mood is affected when his energy bar isn't filled to begin with.