Abstract

The effects of real-world stimuli and how they vary across individuals remains largely unknown for lack of a continuous real-world circadian marker. Here, we show how an underlying circadian rhythm in heart rate (CRHR) can be extracted from several widely used wearables. Analysis of over 130,000 days of data from medical interns on rotating shifts shows how CRHR dynamics are distinct from those of sleep-wake timing and vary greatly among individuals. Analysis of circadian timekeeping in travelers in 5 continents shows how the circadian timekeeping more carefully controls wake time rather than sleep time. We determine a personalized phase response curve (PRC) of CRHR to activity for each individual, representing the first passive and personalized determination of how human circadian timekeeping continuously changes due to real-world stimuli. These results collectively establish CRHR as a practical method to study circadian rhythms in the real world. eTOC BlurbWe show how the circadian rhythm in heart rate can be extracted from real world data collected by wearables. Studying data from a large cohort of medical interns working on shifts, we find very interesting dynamics of this circadian rhythm that are independent of the acute effects of activity or sleep-wake timing. These techniques can also determine personalized parameters of circadian timekeeping.