Has anyone else experienced this, where children don't sleep at night?…

Has anyone else experienced this, where children don't sleep at night? For a week now, our son hasn't been sleeping at all during the night. He plays, doesn't cry, and goes to bed in the morning, then gets up in the afternoon. It's the same routine every day. Naturally, I don't let him sleep during the day after that, but he still stays up all night. I don't know what to do anymore. My husband's sister also used to stay up at night when she was little, drawing and playing. My mother says he needs to see a doctor, but what's the point? We've been to several neurologists, and none have seen any abnormalities.