The circadian rhythm that drives the human sleepwake cycle manifests itself in the variation of the core body temperature (CBT). Measuring CBT continuously, however, is intricate. Heart rate and its variability also varies notably over the course of the day, but is more accessible to measure. We present first results from a study that investigates the biorhythmic relationship between the circadian rhythm of CBT and measures of heart rate variability (HRV). Twelve subjects participated in a 50-hour experiment that was segmented into one hundred 30-minute blocks. In each block, subjects attempted to sleep for ten minutes. CBT and the ECG were continuously recorded. To date, intervals between successive heart beats (RR intervals) were determined for three subjects. From RR intervals of each attempted-sleep episode, power spectral densities (PSD) were estimated with the Lomb-Scargle periodogram and averaged into 0.02-Hz wide bands. The periodogram of each RR-frequency-band time series was estimated and compared against the periodogram of CBT, which, as expected, shows a dominant maximum at a period length of 24 hours. Mutual information, an entropy-based measure of interdependence, was used for the comparison. The highest coherence towards CBT was found for the RR-PSD between 0.24 and 0.34 Hz. This RR-PSD range usually includes the respiratory rate, which may indicate that heart rate is more closely linked to respiration at certain times of the day than at others.
© 2019 by Walter de Gruyter Berlin/Boston
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