Quick answer
Yes, if the air outside is clean, the temperature is between 15 and 22°C, it's not raining, and pollen counts are low. Otherwise, shut them and run a fan.
Why sleep temperature matters
Your body's core temperature drops by about a degree while you sleep. If your bedroom is too warm, your body can't cool down properly, and sleep quality drops fast.
The sweet spot for most adults is 16 to 19°C. In summer, opening a window is often the simplest way to hit that.
What works against it
Pollen. Bedrooms turn into pollen traps if the windows are open during peak hours. If you have hay fever, crack them open in the evening, not at midnight when grass pollen settles back down to ground level.
Air quality. If your local AQI is over 100, especially in winter when traffic emissions sit low to the ground, opening the window means breathing PM2.5 for eight hours straight. Not good.
Noise. A busy street ruins deep sleep. A small fan or white noise machine masks it without needing the window shut.
Security. Ground-floor windows or accessible upper windows need ventilation locks. Pretty cheap, very worth it.
A compromise that usually works
Crack the windows open in the early evening to flush out the day's heat and CO₂. Shut them at bedtime. Let the room temperature do the rest.
Or run a small bedroom fan on low. You get the airflow without the air quality risk.
Check before bed
Open Window Today's "Sleep window" card pulls AQI, temperature, and pollen risk together and tells you whether tonight is a safe one to leave them open.
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