Sleep

Is there some kind of 3 year old sleep regression I didn't know about? Last 4 nights in a row my son has come into bed with us around 3am, sometimes he'll take an hour or so to go back to sleep other times he'll go back pretty quick but then he's awake at 5am. He doesn't nap during the day, except a 20 minute car nap if we happen to be out. He used to sleep in a pretty good routine 8pm to 6am, I know that's less than recommended but it worked for him and he physically would not go to sleep any earlier even if I took him up to bed at 6pm. I don't know what's changed. He has a thicker duvet since the weather has changed so he can't be cold. He tells me the skeletons and ghosts are waking him up. I really don't know what to do.