Many people think sleeping with a slightly open window helps them rest better. The fresh air can feel calming, giving the impression that “the night air itself” helps you drift to sleep. But experts warn that an open window can quietly work against deep, healthy rest.

Outside temperatures often drop suddenly at night. When cold air flows in, your body reacts by tightening muscles and shifting its internal temperature. These changes cause micro-awakenings—“brief, unconscious interruptions”—that make your sleep lighter even if you don’t remember waking up.

Open windows let in pollen, dust, mold spores, smoke, and other irritants. You may fall asleep comfortably but wake up congested or with a dry throat. Moist night air can also change humidity, encouraging mold or irritating your breathing.

Even quiet nighttime sounds—cars, dogs, wind—can interrupt deep sleep. Research shows that noise you “don’t even consciously notice” can raise stress hormones and reduce REM sleep. A closed window acts as a protective barrier.

Open windows can attract insects, animals, and even pose security risks. Cold drafts may cause “stiff neck,” headaches, or tense muscles that prevent full relaxation.

You can keep air fresh without the risks: open windows during the day, ventilate briefly before bed, use a fan, or run an air purifier with a HEPA filter. Maintain a stable room temperature of 16°C–19°C.

Fresh air feels good, but nighttime conditions are unpredictable. Closing the window helps create a stable, quiet, and safe environment for truly restorative sleep.