Human chronotype is determined in bodily cells under real-life conditions

Individuals differ in their preferred timing of sleep and activity, which is referred to as a chronotype. The timing shows a wide distribution; extremely early chronotypes may wake up when the extremely late chronotypes fall asleep. The chronotype is supposed to be determined by the central circadian clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus because the phasing of the pineal melatonin rhythm, which is driven by the SCN, correlates with the sleep timing preference. In addition to the SCN, circadian oscillators are also present in most if not all bodily cells. These peripheral clocks are synchronized by the central SCN clock and by other tissue-specific entraining cues. At the molecular level, the circadian oscillations are based on a complex, self-sustaining mechanism that drives the rhythmical expression of clock genes and their proteins. The aim of the present field study was to elucidate whether the changes in the internal timing of early and late chronotypes, as expressed by changes in the phases of their mid-sleep and melatonin secretion, can also be detected at the molecular clockwork level in subjects examined under real-life conditions. Ninety-five adult volunteers were chronotyped using an adapted Munich chronotype questionnaire to assess their mid-sleep phase, and 6 subjects with early chronotypes and 6 with late chronotypes were chosen for the study. For the assessment of the circadian phase, the subjects provided samples of saliva for the melatonin assay and samples of oral mucosa for the determination of clock gene Per1, Per2, and Rev-erbα mRNA levels every 4 h during a 24-h period. The significant correlation between the phase of the melatonin profile and timing of mid-sleep confirmed the classification of the subjects according to their chronotype. The circadian phases of the Per1, Per2, and Rev-erbα expression profiles in the oral mucosa were advanced in the early chronotypes compared with those in the late chronotypes (p < .001) and correlated significantly with the mid-sleep phase of the individual subjects. Moreover, the circadian phases of the Per1 expression profiles of individual subjects correlated significantly with the phases of their melatonin profiles (p < .05), whereas the correlation for the Per2 and Rev-erbα phases was nonsignificant, although the trend was the same. Our results demonstrate that the individual chronotype in humans living in real-life conditions affects not only the phasing of the daily melatonin rhythm in saliva but also the phasing of Per1, Per2, and Rev-erbα clock gene expression profiles in buccal mucosa cells. This report represents the first demonstration that the human peripheral circadian clock may sense the individual's chronotype under field study conditions. The data contribute to our understanding of the mechanisms underlying human chronotypes in real life.