Random switching, arbitrary intervals, though only *very* rarely in the middle of the night. Actually, I think that happened more before he was fully convinced that he can say "please move over" to me while I'm sleeping and I will. Before that I would occasionally wake up in the morning on the opposite side from the one I went to sleep on. (This was, I might add, far less confusing than the time he closed the bedroom door after I had gone to sleep, causing me to walk smack into it in the middle of the night.)