You probably know the feeling of climbing into bed too late, watching the clock creep past midnight again.
Most of us shrug it off as a bad night and promise ourselves an early one tomorrow.
For women, those stacked habits may do more than make the next day tough.
Women with unhealthy sleep patterns were about 10 percent more likely to develop breast cancer than women with healthy sleep habits.
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That is the main finding of a large study published in BMC Cancer that tracked more than 159,000 women in the UK Biobank.
The researchers followed 159,452 women who were free of breast cancer when they signed up.
During the follow-up years, 7,291 of them developed the disease.
An unhealthy sleep pattern was linked to a 10 percent higher risk of breast cancer compared with a healthy pattern, after adjusting for other factors.
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What counted as unhealthy sleep
The study did not single out one bad habit. Instead, it combined five everyday sleep traits into a single score.
The traits were chronotype, which is whether you are a morning or evening type, sleep duration, insomnia, snoring, and daytime sleepiness. Each trait that fell in the healthy range earned a point.
Seven to eight hours of sleep, an early chronotype and no insomnia counted as healthy.
Women with the lowest scores had the pattern the researchers called unhealthy.
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In practice that meant going to bed late, sleeping too little, and feeling that the sleep was not restful.
What the researchers think is going on
To look under the hood, the team ran a proteomics analysis on blood samples from a subgroup of the women.
They searched for proteins that behaved differently in women with unhealthy sleep and in women who later developed breast cancer.
The signals clustered around three biological systems. Those were inflammation, immune regulation and vascular function.
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In plain terms, poor sleep may push the body's defenses in a direction that leaves breast tissue more vulnerable over the years.
How to read a 10 percent higher risk
A 10 percent relative increase sounds alarming on its own, but context matters.
About 1 in 8 American women will develop breast cancer in her lifetime, according to the American Cancer Society.
A 10 percent lift on top of that baseline is meaningful but modest.
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The study is also observational. It can show that a sleep pattern travels together with higher risk, but it cannot prove that late nights or restless sleep caused the cancer.
Other factors the researchers could not fully measure may still play a role.
Still, sleep is one of the few risk factors a woman can actually change.
The habits the study flagged are easy to look at honestly, a late bedtime, chronic short sleep and sleep that leaves you tired.
Women with new breast symptoms or a strong family history should bring those concerns, and their sleep habits, to their doctor.
This article is made and published by Mie Hermansen, who may have used AI in the preparation.