Dynamics of core body temperature cycles in long-term measurements under real life conditions in women

Studies under real life conditions become more and more relevant in chronobiological and chronomedical research. The present study aims to analyze one of the most prominent biological rhythms: the core body temperature (CBT) rhythm in the real world outside the laboratory. CBT was recorded continuously in 37 healthy women (age between 21 and 44 years, median 29 years) with a newly developed intravaginal temperature sensor for up to 102 days. Sleep logs were available from 23 participants. To quantify the daily dynamics of each individual CBT-curve, novel measurement parameters are introduced which permit the quantification of the phase and shape of the CBT rhythms as well as their relation to the sleep–wake cycle. In addition to the classical phase markers (i.e. nadir and peak), the daily curves were segmented into quartiles by introducing the t25/t50/t75-values which can be used as phase and shape markers. At variance to previous studies, a conspicuous day-to-day variation was shown not only for the time point of the peak, but also for the time point of the nadir. However, the t-values, particularly the t75-value were relatively closely locked to external time and thus represent more reliable phase markers than the nadir. The (variable) time point of the nadir determined the period length, phase and shape of the subsequent CBT cycle. If a nadir occurred close to the wake-up time, the following cycle was considerably shorter than 24 hours, while a nadir distant from the wake-up time was followed by a longer cycle. Thus, the period lengths of the daily CBT cycles of each individual were characterized by an “expand/contract” rhythm. The analyses of the novel phase markers (t25/t50/t75) of the CBT curves allowed to identify “early” and “late” participants who may differ in their phase-response curves with regard to the entraining effect of light. In addition, the novel phase markers mirrored the different social entrainment conditions on weekends and workdays.