My in-laws in northern Japan don't run their AC at night. I don't know the exact reason, but it's not uncommon. They have no screens in some windows, so we couldn't just open the windows either. It was unusually hot and I felt like I was going to die (I woke up feeling like I couldn't breathe and there was no air movement). The screens are necessary unless we want a house full of roaches, mice, lizards, flies, mosquitoes, etc.