In the second half of your cycle, your heart beats faster at night
If you wear a watch or band that tracks your heart rate while you sleep, look at a month of it. The line may rise and fall with your cycle. In 2017 Mohaned Shilaih and colleagues at University Hospital Zurich tested this with 91 healthy women aged 22 to 42, who slept wearing wrist bracelets and estimated ovulation with an at-home urine test.
The team recorded 274 ovulatory cycles. On fertile days, night-time pulse averaged 2.1 beats per minute higher than during the period. It kept climbing, and by the middle of the second half of the cycle it was 3.8 beats higher than on period days.
Alcohol and a big meal before bed raised night-time pulse on their own, and five or more drinks raised it more than the whole cycle did. Even after accounting for them, the wave remained. Most of the women were white and of typical body weight. The study was paid for partly by a company that makes one of the bracelets, and three of the five authors work for it.
The authors see pulse as one sign of the cycle among several, not a way to find ovulation on its own. The study says nothing about using pulse alone to avoid pregnancy. If your watch says your heart beats faster at night in the second half of your cycle, it may simply be your cycle.
Sources
- Shilaih M, de Clerck V, Falco L, Kübler F, Leeners B. Pulse Rate Measurement During Sleep Using Wearable Sensors, and its Correlation with the Menstrual Cycle Phases, A Prospective Observational Study. Scientific Reports, 2017; 7: 1294. doi:10.1038/s41598-017-01433-9