I remember having been worried about that in the past. I have never closed my window since (it has a curtain, though, which is blocking the light at all times). Heating is overrated, a pajama, a blanket and a matress should be enough even for the winter, at least in a Mediterranean climate. What is bothersome is the heat by daytime in any season other than winter, but since I’m alone in my room, being topless is okay.