The study, which tracked 69,000 women not using hormonal contraception, found that average sleep scores drop by 3.4 points in the premenstrual phase. While total sleep duration remains largely stable, the composition of rest changes; deep sleep decreases by 1.8 percentage points, replaced by lighter, more fragmented sleep. US participants recorded the most significant impact, including the lowest sleep scores (68.5) and the highest night-time heart rates among the countries surveyed.

Researchers attribute these physiological shifts to the body’s reaction to progesterone following ovulation. This hormone elevates core body temperature, which hinders the natural cooling process required for deep sleep. The data also clarifies common misconceptions regarding weight fluctuations during this period. While 74% of US women observe an increase on the scale—averaging 0.6 lb—the study confirms this is almost entirely water retention rather than a change in body fat percentage.

In contrast, women using hormonal contraception reported significantly less disruption, with sleep scores falling by only 1.4 points. Livia Robic, a women’s health expert at Withings, notes that understanding these predictable biological shifts can help mitigate unnecessary anxiety for women monitoring their health metrics. The findings, based on data from devices like the ScanWatch 2 and various smart scales, underscore that these shifts are a universal physiological trend rather than a localized health issue.