Abstract
Mobile measures of human circadian rhythms (CR) are needed in the age of chronotherapy. Two wearable measures of CR have recently been validated: one that uses heart rate to extract circadian rhythms that originate in the sinoatrial node of the heart, and another that uses activity to predict the laboratory gold standard and central circadian pacemaker marker, dim light melatonin onset (DLMO). We first find that the heart rate markers of normal real-world individuals align with laboratory DLMO measurements when we account for heart rate phase error. Next, we expand upon previous work that has examined sleep patterns or chronotypes during the COVID-19 lockdown by studying the effects of social distancing on circadian rhythms. In particular, using data collected from the Social Rhythms app, a mobile application where individuals upload their wearable data and receive reports on their circadian rhythms, we compared the two circadian phase estimates before and after social distancing. Interestingly, we found that the lockdown had different effects on the two ambulatory measurements. Before the lockdown, the two measures aligned, as predicted by laboratory data. After the lockdown, when circadian timekeeping signals were blunted, these measures diverged in 70% of subjects (with circadian rhythms in heart rate, or CRHR, becoming delayed). Thus, while either approach can measure circadian rhythms, both are needed to understand internal desynchrony. We also argue that interventions may be needed in future lockdowns to better align separate circadian rhythms in the body.
Highlights
Until recently, assessment of circadian rhythms was restricted to laboratory studies
We found that 8 out of 10 dim light melatonin onset (DLMO) measurements were within the 80% confidence intervals of the heart rate circadian phase, indicating a degree of alignment between the circadian rhythms in the heart and the brain under normal circumstances
When comparing the heart rate and melatonin rhythms from the DLMO data set, we found that 8 out of 10 subjects had a DLMO measurement within the 80% confidence intervals of the phase of circadian rhythm in heart rate (CRHR), when CRHR is shifted by a fixed constant of −4.4 h (Figure 1D) to account for the average phase difference between these markers across the population
Summary
Assessment of circadian rhythms was restricted to laboratory studies. Wearables typically collect data on wrist movement (actigraphy) and heart rate, each of which can separately be used to estimate various outputs of the circadian clock in the body [5,6,7,8,9,10,11]. Bowman et al were able to determine differences in intrinsic circadian timekeeping between individuals [6]. These two methodologies provide powerful tools for the assessment of circadian rhythms in the field
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