It is not bad to leave your window open while you sleep as long as the weather is nice and there's no risks like safety risks, break ins or insects or pollution getting into your room or home while you sleep.
As long as the temperature is warm or cool but not too cold and as long as you live in a pretty safe location and there's no pollution around you or any risks of break ins then it can be good to leave your window open while you sleep.
Leaving your window open while you sleep can allow fresh and cool air into your room and aid your body in it's natural sleep cycles.
Cooler temperatures also help the body to fall asleep faster and help you stay asleep longer and boost your melatonin levels, which can improve your quality of sleep.
Leaving your windows open while you sleep, provides a nice steady flow of fresh air, which can improve your sleep quality, when compared to stagnant air indoors.
And natural outdoor sounds, can also act as a natural soothing white noise, and mask some other disruptive noises.
Also darkness from the outside signals your brain that it's time for sleeping, which can also help to regulate your body's natural sleep and wake cycle.
Although there are some potential drawbacks of leaving your window open while you sleep which include possible allergens and pollutants, bad noise, insects, dryness and safety risks.
Cold or dry air can result in dry nasal passages, which can increase mucus production or even lead to sinus infections.
Open windows can also make for an easy entry point for burglars, especially on the first floor of the home.
Bugs and other insects can also fly into a room with an open window unless it has screens.
And in busy locations and more noisy locations, outdoor noise from neighbors, traffic and wildlife and other animals could disrupt your sleep.
And open windows can also allow allergens like dust and pollen along with outdoor pollutants to enter the home and room, which can trigger or worsen allergy symptoms of asthma symptoms.