Nocturnal heart rate variability across the menstrual cycle and its within-subject coupling with daily premenstrual symptoms: an intensive longitudinal wearable study

Introduction Nocturnal heart rate variability (HRV) reflects cardiac autonomic regulation during the night and may index menstrual-cycle and premenstrual-symptom change. Most prior studies covered a single cycle and used group-average analyses, leaving unclear whether day-level nocturnal HRV is associated with symptoms within individuals. Methods In this intensive longitudinal wearable study, 20 women aged 18–20 years (59 complete cycles; 1,561 cycle-phase-assigned person-days) wore a textile chest-lead electrocardiogram (ECG) sensor and completed twice-daily self-report diaries. The root mean square of successive differences (RMSSD), the standard deviation of normal-to-normal intervals (SDNN), and mean heart rate (MeanHR) were derived from 5-min epochs of nocturnal beat-to-beat (RR) intervals. Cycle phases were estimated from recorded menstrual onsets without hormonal confirmation. Intraclass correlations for the HRV indices ranged from 0.40 to 0.54. Linear mixed-effects models estimated cycle-phase effects and within-subject HRV–symptom associations; participant-level cluster-robust analyses served as sensitivity checks. Results Relative to the follicular phase, the late-luteal phase (the final five days of the cycle) showed higher nocturnal MeanHR (+4.66 bpm, 95% CI [3.77, 5.55]) and lower RMSSD and SDNN (approximately 16% and 11%; all Holm-adjusted p < 0.0001); these contrasts were robust to participant-level inference and to alternative phase and sample definitions, and MeanHR was higher in 18 of 20 participants. Nocturnal HRV in the menstrual phase did not differ significantly from that in the follicular phase. Within individuals, after adjustment for cycle phase, lower RMSSD and SDNN and higher MeanHR were associated with worse self-rated morning physical condition and morning fatigue (16–18 of 19 participants in the expected direction), but statistical support after multiplicity correction depended on the inferential method; no association was detected for mental condition. Evidence that the strength of this association differs between women was inconclusive, and a larger late-luteal change among participants who recalled greater premenstrual worsening at baseline was not supported by participant-level tests. Discussion These findings indicate that nocturnal HRV and heart rate capture menstrual-cycle-related change and covary contemporaneously with day-to-day physical symptoms within individuals; no predictive or diagnostic-accuracy analysis was performed, and confirmation in larger cohorts with prospective symptom assessment and hormonal verification of cycle phase is needed.