In short
- In a 2016 study in student dorm rooms, opening a window cut average bedroom CO2 from 2,585 to 660 ppm, and sleep measured by wrist trackers was better when CO2 was lower.
- A 2023 field study in 29 real bedrooms found less deep sleep and more awakenings at lower ventilation rates in the bedrooms where the difference was clear.
- The WHO strongly recommends keeping night-time road traffic noise below 45 dB Lnight, because noise above that level is linked to effects on sleep.
- The NHS advises people with hay fever to keep windows and doors shut as much as possible when the pollen count is high.
- Cleveland Clinic suggests a bedroom temperature of 60 to 67 F, and a 2012 review found heat exposure increases wakefulness and reduces deep and REM sleep.
- AirNow says AQI values above 100 mean unhealthy air, and the EPA advises keeping windows closed in a clean room during wildfire smoke.
Sleeping with the window open can help your sleep when the air outside is clean, quiet and cool. Small studies have found that sleep was better when bedroom carbon dioxide (CO2) was lower, and an open window is one of the easiest ways to lower it. The catch is everything else that comes in with the fresh air: traffic noise, pollen, smoke and heat or cold can all undo the benefit. The best choice changes from night to night, so it pays to check what's outside before you open up.
Why does bedroom air get stale overnight?
Bedroom air gets stale overnight because you're breathing out CO2 in a small, closed room for hours. With the window and door shut, there's little fresh air coming in to dilute it. In the studies below, CO2 mainly shows how well the room is ventilated. In the 2023 study, for example, humidity rose along with CO2 when ventilation was low.
The difference can be large. In a 2018 study of 17 healthy young adults, average bedroom CO2 was about 1,150 ppm with the window and door closed and about 717 ppm with either one open.
Does sleeping with the window open improve sleep?
Research: early (what this means)A handful of small studies suggest better-ventilated bedrooms are linked to better sleep. The best known is a 2016 study in Indoor Air by Strøm-Tejsen and colleagues. In a pilot experiment, 14 students in single dorm rooms spent a week with the window open and a week with it closed. Average CO2 was 660 ppm with the window open and 2,585 ppm with it closed. In a second experiment with 16 students, a fan in the air intake vent that the students couldn't hear switched on whenever CO2 went above 900 ppm, lowering the average from 2,395 to 835 ppm. Bedroom temperatures didn't differ between conditions.
When CO2 was lower, sleep quality measured by wrist trackers improved, the air felt fresher, and the students reported less next-day sleepiness and better concentration. They also did better on a test of logical thinking.
The 2018 study found that depth of sleep, as people rated it, and one sleep measure from wrist trackers differed significantly between the open and closed conditions. The open rooms were also slightly cooler.
A 2023 study in Science of the Total Environment went into 29 ordinary bedrooms with mechanical ventilation and covertly changed the fan speed, so participants didn't know when ventilation was low, moderate or high. In the 12 bedrooms where CO2 levels showed clear differences, people had less deep sleep, more light sleep and more awakenings at lower ventilation. No differences in thinking tests showed up. At lower ventilation, both CO2 and humidity went up.
These are small, short studies, and the authors of the 2023 study called for larger ones. The direction is consistent, though: fresher bedroom air seems to go with better sleep.
What are the downsides of sleeping with the window open?
The downsides of sleeping with the window open come from what the outside air carries in. Here's a quick guide.
| Factor | When an open window helps | When to keep it shut |
|---|---|---|
| Stale air | Most nights, by lowering CO2 | Rarely a reason to close it |
| Noise | Quiet street, rural area | Traffic, trains, planes or late-night noise |
| Pollen | Low pollen days, after rain | High pollen count, especially if you have hay fever |
| Outdoor air quality | AQI at or below 100 | AQI above 100, or any wildfire smoke |
| Temperature | Cool, comfortable night | Hot, humid night or very cold weather |
Can outside noise ruin your sleep?
Outside noise can disturb sleep, and an open window lets more of it in. The World Health Organization's 2018 environmental noise guidelines strongly recommend keeping night-time road traffic noise below 45 dB Lnight, because noise above that level is associated with adverse effects on sleep. The equivalent night-time limits are 44 dB for railway noise and 40 dB for aircraft noise.
If you live near a busy road, rail line or flight path, an open window may cost you more sleep than the fresh air gains. Opening a window on the quieter side of the house, or opening the bedroom door instead, is one compromise. Our guide to white noise for sleep covers masking sound.
Should you sleep with the window open if you have allergies?
You shouldn't sleep with the window open on high-pollen nights if you have hay fever. The NHS lists keeping windows and doors shut as much as possible among its tips for easing symptoms when the pollen count is high. It says symptoms are usually worse from late March to September, especially when it's warm, humid and windy.
On those nights, a stuffy nose can push you into breathing through your mouth. If that's you, the nasal strips in the Titan Sleep System can make nose breathing feel easier with the window closed. Our guides to hay fever and sleep and air purifiers for sleep have more.
When is outdoor air quality too poor to open the window?
Outdoor air quality is too poor for an open window when the Air Quality Index is high or there's smoke in the air. AirNow explains that an AQI of 50 or below is good, values up to 100 are generally considered satisfactory, and above 100 the air is unhealthy, first for sensitive groups and then for everyone as the number rises.
Wildfire smoke is the clearest case. The EPA's clean room guidance says to close all windows and doors in the room and filter the air with a portable air cleaner. Our guide to sleeping during wildfire smoke walks through the full plan.
Does an open window help keep the bedroom cool?
An open window can help keep the bedroom cool on mild nights, and temperature matters for sleep. Cleveland Clinic suggests keeping your bedroom at 60 to 67 F and says below 60 F is too cold.
Heat can disrupt sleep. A 2012 review in the Journal of Physiological Anthropology found that in real-life conditions, with bedding and clothing, heat exposure increases wakefulness and reduces deep sleep and REM sleep, and humid heat makes this worse. With bedding and clothing, cold exposure didn't change sleep stages, though it did affect heart activity, and the authors suggested cold's impact in real life may be greater than heat's. On hot, humid nights, outside air may not cool the room at all. Our guide to the best bedroom temperature for sleep has more ways to cool down.
How do you get fresh air without opening the window?
You can get fresh air without opening the window by opening the bedroom door, using a ventilation system or running a vent fan. In the 2018 study, an open bedroom door counted as an "open" condition, and CO2 averaged about 717 ppm across the open nights. In the 2016 dorm study, a fan in an air intake vent that the students couldn't hear also kept CO2 much lower, and sleep was better at the lower CO2 levels.
A few practical habits help:
- Check the weather, pollen count and AQI before bed.
- Open the window a little rather than all the way, if noise or cold is an issue.
- Air the room out for an hour before bed and close up when you sleep.
- Keep the bedroom door open if privacy allows.
Can a dry or stuffy nose make window choices harder?
A dry or stuffy nose can make window choices harder, because closing up for pollen or noise can leave the room stuffy, while opening up can bring in allergens. If your nose blocks up at night, you may end up breathing through your mouth whichever you choose.
Nasal strips lift the sides of the nose open from the outside. Mouth tape keeps the lips closed once your nose is clear enough to breathe through all night. The Titan Sleep System pairs the two. Try our nose test first. We don't recommend mouth tape for children.
Check this first. Skip mouth tape and see a doctor if you snore loudly, wake up gasping, have been told you stop breathing in your sleep, or can't breathe comfortably through your nose. More on safety
When should you see a doctor?
See a doctor if you're tired during the day no matter how you set up your bedroom, or if your nose is blocked most nights. The NHS advises seeing a GP if your hay fever symptoms don't improve after taking medicines from the pharmacy.
Loud snoring with pauses in breathing, gasping or choking, or constant daytime sleepiness can be signs of sleep apnea, which needs a proper assessment. If you have asthma or another lung condition, avoid open windows on poor air quality or high-pollen nights and follow the plan you've agreed with your doctor. If you're pregnant, ask your midwife or doctor before trying new sleep products.
Our pick (paid partner): Titan Sleep System
For nights when the window has to stay shut and your nose feels the difference, our pick is the Titan Sleep System. We haven't tested or ranked sleep products ourselves. This is our recommendation, based on how Titan describes it, and it's a comfort aid, not a fix for stale air, pollen or noise.
Titan describes the system as TitanAir Nasal Strips and Titan Mouth Tape together as one nightly routine. The strips are drug-free external nasal dilators with spring-loaded bands that go just above the flare of the nostrils on clean, dry skin. The tape is a bamboo silk strip worn across the lips, and Titan suggests 20 to 30 minutes of daytime practice before the first night.
Titan says its products are third-party tested and certified PFAS-free, with a 30-night guarantee that refunds you if your sleep isn't better after 30 nights.
Read next
- Sleep environment checklist
- Why is my nose stuffy at night in summer?
- Does white noise help you sleep?
Common questions
Is it better to sleep with the window open?
It can be, if the outside air is clean, quiet and cool. Small studies found better sleep when bedroom CO2 was lower, and opening a window is one way to lower it. If opening it lets in traffic noise, pollen, smoke or heat, those can outweigh the fresh air.
Does high CO2 in the bedroom affect sleep?
Small studies suggest it may. In a 2016 dorm study, sleep measured by wrist trackers improved and next-day sleepiness fell when CO2 was lower. A 2023 study in real bedrooms found less deep sleep at lower ventilation rates. The studies are small, and CO2 mainly shows how well the room is ventilated.
Should I sleep with the window open if I have allergies?
Not on high-pollen nights. The NHS advises people with hay fever to keep windows and doors shut as much as possible when the pollen count is high, and says symptoms are often worse from late March to September. A bedroom door left open or a quiet ventilation fan can bring in air instead.
Is it bad to sleep with the window open in winter?
Not necessarily, as long as you stay warm. A 2012 review found that with normal bedding and clothing, cold exposure didn't change sleep stages, though it did affect heart activity during sleep. Cleveland Clinic says a bedroom below 60 F is too cold, so a slightly open window may be enough.
How do you ventilate a bedroom without opening the window?
Opening the bedroom door helps. In a 2018 study, an open window or an open bedroom door lowered average CO2 from about 1,150 to 717 ppm. In a 2016 study, a quiet fan in an air intake vent kept CO2 lower and sleep improved. A ventilation system or a vent fan can do the same job.
Should you open the window if the air quality is bad?
No. AirNow says AQI values above 100 are unhealthy, first for sensitive groups and then for everyone. During wildfire smoke, the EPA advises keeping windows and doors closed and filtering the air in a clean room. Check the AQI before you open up for the night.
Sources
- The effects of bedroom air quality on sleep and next-day performance
- Window/door opening-mediated bedroom ventilation and its impact on sleep quality of healthy, young adults
- A single-blind field intervention study of whether increased bedroom ventilation improves sleep quality
- Environmental noise guidelines for the European Region
- Hay fever
- AQI Basics
- Create a Clean Room to Protect Indoor Air Quality During a Wildfire
- The Best Temperature for Sleep
- Effects of thermal environment on sleep and circadian rhythm
- Titan Sleep System
About this article
- Research
- 10 published sources, listed above.
- Evidence
- Early. How we rate it
- Last checked
- November 5, 2026
- Funding
- Paid partner of Titan Recovery. Their ads are labeled.
- Found an error?
- Tell us. We log fixes on the corrections page.
* This content is for informational and educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any disease or health condition. Talk to a qualified healthcare provider about your health.