Because this used to happen to me a lot. I'd go to sleep with a window open or closed and wake up in the morning with it closed or open. I always used to think it was my parents but I opened the window for the first time since I moved out and started living in my own flat and I woke up this morning to find it closed. I figured the wind could have blown in closed but in opens inwards and from the top so I can really see it happening. Am I sleep closing my window if I get too cold? Can people do stuff like that in their sleep?