Sleeping with the windows open and turning off your air conditioner seems like a great way to save energy. It makes sense to give your HVAC a break when the sun sets and the temperature drops, doesn’t it? Unfortunately, that isn’t always the case. One of the ways you might be unknowingly driving up your utility bill is by sleeping with the windows open at night. Your air conditioner has worked hard to cool your home, but if you leave your windows open all night, it has essentially undone all its hard work. The AC system removes heat and humidity from the indoor air during the day. The system will have to work harder to restore your desired temperature if the outside air temperature is even one degree warmer than the inside air temperature. Additionally, the outside air contains more humidity than the inside air, making your rooms feel stuffy and uncomfortable. The presence of humidity makes temperatures appear warmer than they actually are. In the event that your home becomes too damp, you might feel inclined to set your thermostat even lower to compensate. Sleeping with open windows has been shown to be beneficial in some studies. You can crack a window overnight if the outside temperature is about 70 degrees and the humidity is low. You should, however, close them as early in the morning as possible to prevent outside moisture and sunlight from increasing your internal temperature.