Nocturnal heart rate runs 1 to 2 bpm higher in winter across 62,683 Ring members, and reverses below the equator

Seasonal rhythms in sleep and cardiovascular physiology are a known story in sleep medicine, but most of the evidence comes from sleep labs of a few hundred patients on a single night. A retrospective study of 2,386 home sleep studies in Fresno, California reported a winter mean nocturnal heart rate of 68.3 bpm against a summer mean of 65.7 bpm, a 2.6 bpm gap (p less than 0.001), with apnea burden higher in winter and sleep latency longer in summer (Vazquez et al., 2026). That paper leaves three questions open: does the same gradient hold across continents at consumer-wearable resolution, is the effect from a multi-night wearable read consistent with the single-night lab measurement, and does the direction reverse cleanly in the southern hemisphere as a pure seasonal-physiology account would predict.

We pulled every Ultrahuman Ring member with at least 30 valid nights in both the December 2024 to February 2025 window and the June to August 2025 window, then computed each member’s mean nocturnal heart rate per season and the within-person winter-minus-summer delta. Region was assigned from each member’s timezone; 62,683 members across five regional clusters cleared the bar, from Australia at 1,351 to the Americas at 36,537.

Every northern region shows a winter rise in nocturnal heart rate, and Australia, the only large southern-hemisphere cluster, flips the sign. Europe ran +1.88 bpm higher in winter (95% CI +1.82 to +1.93); the Americas +1.42 bpm (CI +1.39 to +1.46); Asia outside India +1.28 bpm (CI +1.19 to +1.37); India +0.98 bpm (CI +0.85 to +1.10). Australia ran −1.08 bpm on the same calendar (CI −1.26 to −0.89), meaning higher in its own winter.

Figure 1. Mean nocturnal resting heart rate by region and seasonal window (December 2024 to February 2025 versus June to August 2025), in 62,683 Ultrahuman Ring members with at least 30 valid nights in each window. Region assigned from each member’s recorded timezone; Australia, the only large southern-hemisphere cluster, shows the predicted sign-flip relative to the four northern regions.

The age gradient sharpens the read. Within every northern region, the winter-minus-summer delta broadly shrinks with age – largest in under-30s and smaller in older bands – with the same broad compression in the Americas and non-Indian Asia. In Europe the 18-to-29 cohort ran +2.31 bpm and the 60-plus group ran +1.12 bpm. The secondary markers move in concert: overnight heart-rate variability (HRV) ran 2 to 5 ms lower in winter in most regions, total sleep ran 16 to 27 minutes longer, time awake after falling asleep ran 5 to 7 minutes longer, and sleep efficiency dropped about a percentage point, so winter sleep is slightly longer but slightly more fragmented. The wearable effect is roughly half the lab gap, which the within-person multi-night mean and the clinically referred lab sample would both predict; the paired design holds age, sex, baseline fitness, and Ring fit fixed automatically, so the cross-region comparison sits downstream of each member’s own baseline.

Members carry a real seasonal baseline in the data their Ring already collects: nocturnal heart rate sits a beat or two higher in winter, and the rise tracks the calendar of each member’s own hemisphere rather than the month on the clock. A drift of one to two beats across the cold months is ordinary.