This is widely known and a helpful tip if you have troubles falling asleep or have insomnia.Before you are going to bed, just open your bedroom windows for 5-10 minutes to let in fresh and cool air. The usual room temperature of a room is around 24 Celsius/75.2 Fahrenheit degrees which is a little too warm for the body when you're about to go to bed.The perfect room temperature when falling asleep is 18 Celsius/64.4 Fahrenheit degrees which is achievable if you open your windows.I wouldn't recommend having your windows open all night though, as for me I always wake up in the middle of the night because it's too cold...