That probably means there's either something in the way of the bed, it has been claimed by someone else, or there's a tv or radio on in the room. Sometimes I miss a radio until I've tried to send my Sim to bed a couple times.
If it's none of those, try replacing the bed. I haven't seen many item glitches in Sims 4, but it happens in Sims 2 sometimes and replacing the item fixes it.
Occasionally, it's just that they think another need is more important and it keeps waking them up. In cases like that, they can get stuck in a loop of trying and failing to do both because they are both so low. If that's the issue, there may be no way to fix it aside from cheating them up.
This thread is really old and will likely be closed before I even see a response from you, so I hope one of those works.
