Once in a while I'll get a sim doing that, there's no reason for it, and I have to exit and reload to make it stop happening.
But there are things that will wake sims up. The most common is a radio in the bedroom. I've made it a rule, no radios in the bedrooms, because of this. You can just turn it off, sure, but sometimes other household members will decide they HAVE to dance to THAT PARTICULAR radio, and come in and turn it on.
Pets, if you have them. This is a behavior you can train out of them, where they purposely wake someone up. If your sim wakes up and their pet is nearby, see if they have the option "Lecture not to Wake Up."
Other sims practicing instruments in the same room. Pretty sure this is what is meant by "justifiable homicide." We need someone to create an animation of someone being beaten to death with their own violin. In what universe is it okay to walk into your sibling's room and start practicing?
I do wish a sim would wake up with a thought/speech bubble indicating WHY they woke up without a full Energy bar though!
