As @Cupid suggests, it could very well be Pets interrupting your sims sleep. Mine did the same until I locked the bedroom door for pets. Another possibility are their moods, I noticed certain moods will cause them not to sleep, I'm not entirely sure about adults as it hasn't occured in a while but especially for children when they are awoken by the monster under the bed, they will not go back to sleep unless SOMEONE else is awake to comfort them. Perhaps have your sim perform another action for a bit and then try to see if they sleep - it could be a glitch in that specific save or even CC if you use it.