Night heart rate shifts with the menstrual cycle, a months-long wearable study finds
Twenty young women in Japan wore a chest sensor in their sleep for about four months. In the last days before a period, their night heart rate ran almost 5 beats per minute higher and heart rate variability ran lower, and rougher nights tended to come before rougher mornings.
Soma Labs3 min read
If you track heart rate variability (HRV) or resting heart rate overnight, you may have seen your numbers drift for no obvious reason. For people who menstruate, a new study out today puts one likely reason on firmer ground: the cycle itself, measured night after night inside the same person.
Researchers at the University of Osaka followed 20 young women for a median of 126 nights each. In the final days of each cycle, their night heart rate climbed and their HRV fell, and the pattern repeated cycle after cycle.
The setup
The team, led by Ken Kiyono and Li Li at the Graduate School of Engineering Science, recruited women aged 18 to 20 at a single institution. Anyone using hormonal contraception over time was excluded. Twenty women made the final analysis, contributing 59 complete cycles and about 1,560 nights that could be matched to a cycle phase.
Instead of a watch or ring, participants wore a seamless garment with a small clip-on sensor that recorded a chest-lead ECG, which measures each heartbeat directly. In twice-daily diaries they rated their physical condition, their mental condition and how tired they felt.
Cycle phases were counted back from the start of each period. Hormones were not measured, so ovulation dates were estimates.
What the study found
Compared with the weeks after a period ended, the last five days of the cycle looked different overnight:
- Night heart rate was 4.66 beats per minute higher on average (95% CI 3.77 to 5.55). It rose in 18 of the 20 women.
- RMSSD, a common HRV measure, was about 16% lower, and SDNN about 11% lower.
- Nights during a period itself did not differ clearly from the post-period baseline.
The researchers then looked within each woman. After accounting for cycle phase, nights with lower HRV and higher heart rate tended to come before mornings she rated as physically worse and more tiring. That direction held for 16 to 18 of 19 women. No link showed up for mental condition ratings.
What to keep in mind
Small and narrow. Twenty Japanese women aged 18 to 20 from one institution, all off hormonal contraception. The authors say the results should not be assumed to apply to older women, contraceptive users, athletes or people with irregular cycles.
Estimated phases. Without hormone tests or ovulation kits, the phase boundaries are approximate. The late-cycle window is the most reliable because it is anchored to the next period's start.
Mixed statistical support. The cycle-phase shifts were robust across every check. The night-to-morning link was consistent in direction, but whether it survived correction for multiple tests depended on the method, so the authors call it something that needs confirmation. Some analysis rules were also set during peer review.
Funding and ties. The work was funded by Japan's JSPS KAKENHI grants. One author, Li Li, was employed by IntaSect Communications, a Tokyo company.
Where Soma fits
The Mind score in the Soma app, which is not yet available, reads HRV, resting heart rate and breathing from Apple Health. This study is a reminder that a lower HRV week is not automatically a bad sign. For many people, the menstrual cycle is part of the normal background, and the most useful comparison is against your own pattern over time, cycle by cycle, rather than against yesterday alone.
Sources, as of November 9, 2026: Syarifah Nabilah Syed Taha, Li Li, Ken Kiyono and colleagues, "Nocturnal heart rate variability across the menstrual cycle and its ...," Frontiers in Digital Health, doi 10.3389/fdgth.2026.1992950, published under CC BY 4.0. This post describes research and isn't medical advice.