Day & night reversed

Jin has taken to waking up at 2am and staying up for 2 hours! He seems to think that it's play time or something. He sleeps more during the day than night :( I don't know how he got into this habit but it just ain't right. What can you do if he just doesn't want to sleep? We have no choice but to stay up with him, we don't play with him, we just hold him, it's the only thing that keeps him quiet. He used to be such a good sleeper, 8:30pm through to 2 or 3 am! What happened!?!