Key result
Apple Watch shows strong agreement with EKG monitors for HRV tracking across menstrual cycles.
Why the study?
Heart rate variability variation across the menstrual cycle and the accuracy of consumer-grade wearables in measuring it are underexplored.
Does HRV vary across the menstrual cycle, and does the Apple Watch accurately measure HRV compared to an EKG-based monitor in healthy young females?
Observational (n=13)
Does HRV vary across the menstrual cycle, and does the Apple Watch accurately measure HRV compared to an EKG-based monitor in healthy young females?
The Apple Watch provides HRV measurements comparable to an EKG-based chest monitor, and HRV does not significantly vary across menstrual cycle phases in healthy young females.
Supports device agreement for HRV in small female cohort; leaves open clinical utility and cycle effects in larger populations.
PURPOSE: Heart rate variability (HRV) is a metric reflecting autonomic nervous system and cardiac function that is rapidly gaining popularity in the United States, but its variation across the menstrual cycle and the accuracy of consumer-grade wearables in measuring it are underexplored. This study evaluated differences in HRV across menstrual cycle phases and compared the Apple Watch to an EKG-based, Polar HR monitor assessments of HRV across the menstrual cycle in young adult females. METHODS: Thirteen healthy females (ages 19-28, regular menstrual cycles) underwent HRV measurement (standard deviation of NN intervals, SDNN) every 4 days across one menstrual cycle (~28 days, 7 visits) during 5-minute seated rest periods. HRV was recorded simultaneously using an Apple Watch, with integrated mindfulness reading for HRV assessment, and a Polar HR monitor and connected software (Elite HRV app). Menstrual cycle days were determined via self-reported onset of menstruation. Two-way ANOVA (time × device) assessed differences in HRV across phases and devices (p<0.05). Bland-Altman analysis was used to further evaluate device agreement. RESULTS: Mean SDNN values from Apple Watch across measurement days were 81.3 ± 43.2 ms (day 2), 82.4 ± 45.1 ms (day 6), 87.5 ± 43.8 ms (day 10), 73.2 ± 53.6 ms (day 14), 108.0 ± 61.2 ms (day 18), 90.8 ± 63.0 ms (day 22), and 90.5 ± 72.1 ms (day 26). Corresponding Polar HR monitor values were 82.9 ± 36.4 ms (day 2), 81.0 ± 31.6 ms (day 6), 81.3 ± 28.8 ms (day 10), 78.4 ± 48.6 ms (day 14), 92.7 ± 43.6 ms (day 18), 68.7 ± 28.9 ms (day 22), and 72.7 ± 23.9 ms (day 26). Two-way ANOVA revealed no significant effects of time ( *P* = 0.08), device ( *P* = 0.64), or their interaction ( *P* = 0.74). Bland-Altman analysis indicated strong agreement between Apple Watch and Polar HR monitor measurements (mean difference 8.1 ± 10.4 ms; 95% limits of agreement -12.4 to 28.5 ms). CONCLUSIONS: In our sample, HRV did not significantly vary across menstrual cycle phases, and the Apple Watch SDNN values were similar to the EKG-based Polar HR monitor (P>0.05). These findings support the use of consumer-grade wearables for HRV monitoring in females across the menstrual cycle, enhancing accessibility for health tracking. Studies with hormonal phase confirmation are needed.
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Bergfeld et al. (2026) conducted an observational in Healthy (n=13). Apple Watch vs. Polar HR monitor was evaluated on Heart rate variability (SDNN) differences across menstrual cycle phases and devices (MD 8.1 ms, 95% CI -12.4 to 28.5, p=0.64). The Apple Watch and EKG-based Polar HR monitor showed strong agreement for measuring heart rate variability across the menstrual cycle (mean difference 8.1 ms; 95% limits of agreement -12.4 to 28.5).
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