The cough that won't quit, the pillow flipped a dozen times, the alarm that goes off before you've really slept.
Most of us put it down to a cold coming on, a dry room, or just a bad night.
A new piece of research from the University of Cambridge points to a less obvious trigger. The air outside your window.
Researchers from Cambridge, Tsinghua University and the Swedish sleep tracking firm Sleep Cycle used phones to count night-time coughs in 32 cities across 12 countries.
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When daytime fine particle pollution went up, so did the coughing a few hours later in bed.
The work was published in Communications Health on September 15, 2026.
It drew on data from tens of thousands of users over 18 months, in countries from Australia and Brazil to Japan, Sweden and the United States.
How the researchers counted coughs around the world
The team used the Sleep Cycle app, which listens for sounds through the night.
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Coughs were detected and counted on each phone itself, so no audio ever left the device.
Only anonymous, city level cough counts were sent for analysis.
That let the team match nightly cough counts in each city to that day's level of PM2.5.
Those are fine particles, less than 2.5 micrometers across, that come from traffic, wood burning and industry.
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They are small enough to slip past the nose and throat and reach deep into the lungs.
For every rise of 10 micrograms of PM2.5 per cubic meter of air, night-time coughing went up by about 2.4 percent.
The link held even in cities where pollution stayed below the World Health Organization's recommended daily limit.
It also held after the researchers accounted for weather and the flu season.
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"This suggests that there may be no such thing as a safe threshold for exposure to air pollution," said Professor Cecilia Mascolo of the University of Cambridge.
She added that the research is purely observational.
Observational means the study can show a link, not prove one thing caused the other.
But the signal was consistent across very different cities and climates.
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A mild cough still matters for your sleep
Most research on dirty air tracks the heavy end. Hospital visits, asthma attacks, early deaths.
This study picked up the quieter signal most people shrug off.
A cough wakes you up, or it stops you from sinking into the deeper stages of sleep.
Even a handful of brief wake-ups a night can leave you groggy the next day.
Over months, broken sleep has been tied to higher blood pressure, mood changes and weaker focus.
Dr Tong Xia of Tsinghua University, who led the analysis, said coughing is more than a nuisance.
A cough can be a sign of irritation or early inflammation in the lungs, she said, and in some people it may be the first warning of something that could later turn chronic.
What you can do about the air in your bedroom
You cannot filter a whole city on your own. But you can nudge the air in the room where you sleep.
The US Environmental Protection Agency names three basic ways to improve indoor air.
Cut the sources, improve ventilation, and use an air cleaner.
In a bedroom that usually means not burning candles or incense before bed, and keeping cooking smoke and wood smoke out of the room.
A portable air cleaner with a HEPA filter can trap most of the fine particles the size of PM2.5.
Research on HEPA air filters at home has found measurable drops in indoor particle levels within hours of switching one on.
Checking a local air quality index before bed is another small step.
On days when outdoor PM2.5 is high, keeping the bedroom window closed and letting a filter work through the night tends to beat airing the room out at the worst moment of the day.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.