Estimation of melatonin level and core body temperature: heart rate and heart rate variability as circadian rhythm markers

Abstract

ABSTRACT Because the circadian rhythm is different for each individual depending on what light environment they are exposed to on that day, the circadian rhythm needs to be constantly managed in everyday life. However, the typical circadian markers, changes in melatonin (MLT), and core body temperature (CBT) are difficult to measure 24-hour cycle of daily life. In this study, to propose relatively easy-to-monitor cardiac rhythmicity-related indicators as circadian markers, the biomarkers, including MLT, CBT, heart rate (HR), and heart rate variability (HRV), were collected from 22 Korean men. The MLT and CBT showed the largest positive correlation with R wave-to-R wave (RR) interval (Pearson’s r = .620, p < .001) and HR (Pearson’s r = .476, p < .001), respectively. In addition, the difference between the peak times of the seventh-order fitted MLT (adjusted R2 = .960, peak time 04:56) and RR interval (adjusted R2 = .960, peak time 04:35) was approximately 21 min; especially, the nadir times of the fitted CBT (adjusted R2 = .961, nadir time 04:55) and HR (adjusted R2 = .934, nadir time 04:52) were almost the same. Therefore, we can confidently say that HR and RR intervals are good indicators for estimating an individual’s circadian rhythm.