New Research Reveals Hidden Long-Term Rhythms in Resting Heart Rate

What's new

New research, drawing on extensive smartwatch data, indicates that resting heart rate (RHR) exhibits distinct long-term patterns, or 'infradian rhythms,' that are independent of external daily cycles. While the 24-hour circadian rhythm is widely recognized for governing sleep and wakefulness, this study highlights slower, more extended cycles in RHR, which can range from weekly to multi-month periods. These findings challenge the notion of a fixed physiological baseline and suggest that RHR, a metric linked to stress, fatigue, and fitness, is influenced by these previously uncharacterized internal clocks [1].

The study, published in EBioMedicine, analyzed data from over 600 college students, collected over several years through smartwatches. Researchers found that approximately 70% of participants displayed a dominant RHR rhythm. These rhythms caused RHR to fluctuate by as much as 15 beats per minute from peak to trough within their respective cycles. While individual rhythms varied, three general categories emerged: weekly (13% of participants), monthly (37%), and longer 'multi-month' cycles (50%) [1].

The science behind it

Human biological time refers to the body's intrinsic systems for measuring time, distinct from external clocks. While the circadian rhythm is the most prominent, longer cycles, known as infradian rhythms, are also observed in nature, such as the female reproductive cycle or animal migration patterns. These rhythms are governed by internal 'clocks' within the brain and other organs, influencing various bodily functions [1].

Detecting infradian rhythms in humans is challenging due to their extended duration and individual variability, requiring years of continuous data. The researchers utilized spectral analysis, a technique akin to splitting light into a spectrum, to identify the dominant rhythmic 'color' for each participant's RHR data. This method allowed them to uncover these slower oscillations that would otherwise be obscured by daily fluctuations [1].

Interestingly, some of these RHR rhythms showed alignment with environmental cues. For instance, yearly cycles often correlated with seasonal changes, with lower RHR observed in summer. Weekly cycles tended to align with weekend schedules. While monthly RHR cycles in females often coincided with menstrual periods, a significant portion (20%) of males also exhibited monthly rhythms, suggesting that these patterns are not solely attributable to reproductive hormones. The study posits that biological rhythms are designed to synchronize with the environment, with RHR cycles potentially locking onto cues like light levels or social schedules, though internal mechanisms can also drive these rhythms independently [1].

What it means in practice

The discovery of these diverse infradian rhythms in resting heart rate has significant implications for understanding individual health and well-being. Recognizing that RHR naturally fluctuates over weeks or months, rather than remaining static, could lead to more personalized health monitoring. Instead of comparing an individual's RHR to a population average, future health assessments might consider their unique rhythmic baseline and deviations from it [1].

For healthcare, understanding these rhythms could enhance the prediction and management of health conditions. Previous research has explored how infradian rhythms impact symptoms of epilepsy, heart attacks, and psychiatric episodes. By tracking an individual's specific RHR cycles, it might be possible to identify periods of higher risk or to tailor therapeutic interventions more precisely. This approach moves beyond a one-size-fits-all model, acknowledging the body's inherent temporal complexity [1].

Furthermore, the findings highlight the potential of wearable technology, like smartwatches, to democratize and accelerate research into human biology at scale. The ability to collect continuous, long-term physiological data from large populations is crucial for uncovering subtle, extended biological patterns that were previously difficult to observe. This opens new avenues for investigating how these slowly oscillating heart rates might correlate with changes in mood, stress levels, and disease susceptibility [1].

Caveats

While the study provides compelling evidence for infradian rhythms in resting heart rate, it's important to note that the research was conducted on a population of college students. This demographic might not fully represent the broader population, which could have different lifestyle patterns, age-related physiological changes, or health conditions that influence RHR rhythms. Therefore, further research across diverse age groups and health statuses is needed to confirm and expand upon these findings [1].

The study also identifies correlations between RHR rhythms and environmental cues like seasons or weekly schedules, but it emphasizes that these cues act as 'nudges' rather than the sole cause. The exact underlying biological mechanisms driving these infradian rhythms, particularly those not directly linked to obvious external factors or known hormonal cycles (like monthly rhythms in males), remain to be fully elucidated. More research is required to understand the internal biological clocks and pathways responsible for these long-term oscillations [1].

Source: https://theconversation.com/data-from-more-than-600-people-shows-resting-heart-rates-have-their-own-secret-rhythm-290635