Three in Four Women Sleep Worse Before Their Period, Study Finds

A massive new analysis of sleep-tracker data confirms what many women have long suspected: the week before a period is measurably worse for rest. Three in four women (76%) sleep worse before their period, according to a Withings analysis released October 15, 2026. The research drew on nearly half a million menstrual cycles logged by 69,000 women across 11 countries, making it one of the largest examinations yet of how the menstrual cycle shapes sleep.

The headline numbers are striking. Average sleep scores fall 3.4 points compared with the days just after a period, while night-time heart rate climbs in nine out of ten women by 2.8 beats per minute on average. Deep sleep shrinks by 1.8 percentage points and is replaced by lighter sleep, while heart-rate variability, a key marker of overnight recovery, drops by 3 milliseconds. Notably, women sleep only about five minutes less during that week. The loss is in quality, not quantity. In short, women sleep worse before their period in ways a wrist-worn sensor can now measure night after night.

A global pattern, with American women hit hardest

The dip showed up in every country studied, though starting points differed. Women in the United States fared worst of all: their pre-period sleep score sank to 68.5, the lowest recorded, their nights were the most fragmented at 5.4 interruptions, and their night-time heart rate reached 68.6 beats per minute, the highest in the dataset. Finland scored best at 74.1, while Denmark saw the steepest single drop at 4.0 points. British women recorded the third-highest night-time heart rate at 66.1 beats per minute, behind only the US and Australia, as South West News reported. Statistically, though, no country's decline differed significantly from the rest. As the researchers put it, this is a shared pattern, not a national one; what differs is where women start.

Hormonal contraception appears to blunt the effect considerably. Among roughly 6,800 women using hormonal methods, sleep scores fell by only 1.4 points in the pre-period week, less than half the decline seen in natural cycles, and night-time heart rate rose by 1.0 beat per minute rather than nearly three. The drop in heart-rate variability nearly vanished. Because most hormonal methods smooth out the natural rise and fall of progesterone and estrogen, the company says the contrast supports a hormone-driven explanation.

Hormones, heat, and why the scale lies too

The biology behind the restless nights is well established. After ovulation, the body produces progesterone, which raises core body temperature and shifts the nervous system toward a more alert state, pushing heart rate up and recovery down. Because the body needs to cool down to enter deep sleep, a warmer body tends to sleep more lightly. The new data fits that picture precisely: the decline began just after estimated ovulation and bottomed out three to five days before the period, and it ran deeper in cycles with confirmed ovulation.

Independent lab research points the same way. Research from SRI International in California has shown that sleep quality deteriorates before the period and that REM sleep falls in the second half of the cycle, while a Columbia University study found women's heart rates ran 2.3 beats per minute higher, with lower heart-rate variability, in the second half of the cycle than in the first. What the new analysis adds is scale: hundreds of thousands of real-world nights measured continuously, rather than small lab cohorts.

The study also tackled a familiar frustration: the period-week weight gain. During menstruation, 74% of American women weighed more than in the days just after, by an average of 0.6 pounds. Yet body-fat percentage did not move at all, while body water rose by about 0.4 pounds, meaning more than half of the gain on the scale is fluid. The cyclical swing ran 0.5 to 0.7 pounds in every country. In other words, the scale is telling the truth about water, not fat.

What the numbers can't tell you

Scale is also this study's main caveat. The findings come from an observational, within-woman analysis of anonymized data from Withings device owners, not a peer-reviewed journal paper, and the company acknowledges its users may not represent the general population. Cycle phases were estimated from logged period dates, and the results describe associations, not proof of cause. Independent experts were not quoted in the release, so outside validation will matter as researchers digest the full dataset, which the company says it has published alongside its Menstrual Cycle Report.

Still, the practical message lands. "Looking across 469,000 cycles, the same pattern appears again and again," said Livia Robic, women's health expert at Withings. "Knowing the pattern is normal and predictable can help women make sense of a restless night or a heavier morning on the scale, rather than worry about it." For young women already tracking sleep on a watch or ring, that normalization may be the most useful finding: a rough pre-period week reflects physiology, not personal failure.

There are modest steps that align with the mechanism. Sleep scientists generally recommend a cool, dark bedroom, since the body must shed heat to reach deep sleep, along with a consistent schedule through the cycle rather than weekend catch-up. Comparing each night against your own baseline matters more than chasing a fixed score, particularly during the days when women sleep worse before their period. This finding also echoes other recent sleep research, including a study linking heavy social media use to worse student sleep (Social Media Hurts Student Health Through Worse Sleep) and a trial in which paying students to sleep more raised their grades (Getting Paid to Sleep Raised Students' Grades). And the release carries the standard caution: it is information, not medical advice, and anyone concerned about their sleep, cycle, heart rate or weight should consult a healthcare professional.

Withings says the research is already feeding its products: its Health Assistant now draws on two studies of anonymized member measurements to answer questions about cycle-linked sleep dips and how readings compare with other women at the same life stage. Whether or not you wear the hardware, the underlying insight travels well beyond one brand's app: the menstrual cycle is a whole-body event, and sleep is one of the places it shows up first.

According to the Withings analysis, the next step is more research into how these nightly shifts connect to daytime symptoms, an area the company says remains underexplored despite affecting half the population for decades of their lives.