My sims have a relaxed schedule, not strict at all but with a few guidelines. I always want them to get a full night's sleep and have a full breakfast, plus max their fun (or close), before work or school, so I don't let them stay up too late. Other than that, I generally have them skilling in the evening and stop when their moods drop and they're not learning quickly anymore. If it's too early for bed, i.e. they'll wake up too early, I'll have them do something fun for a little while instead, like play with a toy or read in bed.
The only other guideline is that when I'm playing a couple, I always have them go to bed together if feasible. Occasionally it doesn't work out, but that's the goal and they generally stay on the same schedule.