You’re going to struggle to get to sleep if it’s too humid, for example. If you open your window, you might then be too cold. Perhaps your duvet is too light or heavy, or maybe the neighbors are having a backyard party on a Saturday night.
Sleep deprivation among adults is such a big issue that even official governmental bodies are trying to combat it. The NHS recommends use shades or reflectively materials on the outside of windows, and if you can’t do that, then get light-colored curtains.