Women have higher heart rate and heart rate variability in phase dead zone
In our analysis cohort, we found that women had a consistently higher average HR in sleep and across the daytime as compared to men, overlapping only in the evening (Figure 1a). Interestingly their HRV seemed to undergo shifts across sleep (12-6 a.m.) being lower, increasingly sharply around 7 a.m., and remaining higher across the day until bedtime (Figure 1b). Average HR in the morning (9 a.m.) was significantly higher in women as compared to men, whereas HRV was significantly higher between 9 a.m. to 3 p.m. between the sexes.
Figure 1: Time-series line curves of changing average HR (left panel) and HRV (right panel) across all countries for male and female users. N=165 indicating the country's gender-average at each time point. *,**, *** denote p<0.05, 0.01, 0.001 respectively. Error bars represent S.E.M.
Men and women have different times of HR peaks in the day: Analysis across countries
Since the SRS relies on HR synchrony with the diurnal circadian cycle, we looked to see across the world the most frequent times of the day at which the lowest and highest HR was registered. Interestingly, we found that among men, highest HR was reported in the evening in the majority of the countries, in the phase delay period (Figure 2a). Women displayed a less distinct pattern of HR maxima with different clusters of countries reporting HR maxima at different times. On deeper analysis, we found no discernible geographic pattern relating women of specific countries reporting HR maxima at specific times. Lowest HR was reported at phase advance, deep sleep time as expected from sleep data for both men and women alike (Figure 2b).
Figure 2: Graphical representation of country wise counts of highest (left panel) and lowest (right panel) heart rate in men and women.
Conclusions, Limitations and Future Directions
Subtle differences between men’s and women’s resting HR are well documented, primarily attributed to the larger size of male bodies. Generally, women’s HR is found to be 5-6 bpm higher than men’s 5. HRV in males has also been reported to be generally higher than that of women in nominally “healthy” individuals 6. Another fundamental difference between genders is their reliance on specific arms of the autonomic nervous system (ANS) to regulate HR and HRV. Females have been shown to have a greater reliance on the parasympathetic arm of the ANS to regulate vagus nerve tone that regulates heart output 7. This reliance has a strong hormonal basis, which diminishes with age and is likely a significant predictor of women developing cardiovascular disease. However, most HR and HRV data reported has been of combined day-wise averages which loses the temporal information critical to finding associations between lifestyles and these parameters.
Wearables with near-continuous PPG-derived HR measurements can bridge the gap between resting HR, sleep HRV, and daytime patterns. The insights gained from mapping lifestyle factors, such as professional and personal activities, are invaluable for optimising health and longevity. For instance, our analysis revealed that a larger proportion of men reach their peak HR in the evening, a time when elevated HR can be detrimental to sleep and recovery. Women, on the other hand, exhibited more variable HR peaks but generally maintained their stress exposure within the circadian dead zone. This pattern likely contributed to an improved overall Stress Rhythm Score (SRS) for women in our previous country-level analysis which relies on dynamic HR values.
It is important to note that the conclusions derived from this analysis are solely based on HR recorded from the Ultrahuman Ring AIR. The fact that we didn’t find any continental or hemisphere-centric trends in HR maxima at specific times of the day for women, underscores the need for additional lifestyle related information. As mentioned earlier, the user base of Ultrahuman is largely professionals, athletes and active individuals with few clinical conditions; the results are representative of that particular demographic profile. Consequently, there is limited generalizability of these results to a global population. It is equally possible that women are consciously aligning their activities to circadian cycles or that they are influenced more by familial commitments that are restricted to specific times of the day.
The evidence that disrupted diurnal rhythms and modern lifestyle diseases is largely related to sleep-wake cycles. This is limited research into the benefits of aligning circadian rhythms for overall cardiovascular health. Research abounds in animal models to support this notion. There is a critical need to gather more real evidence around this, keeping in mind gender- and age-related demographic changes, to better tailor evidence-based prevention and perhaps intervention as well 8.
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