Under such conditions where it is allowed to consider only the individual’s “feeling best” rhythm for planning the day, such as on free days in real life or in constant routine at a sleep research laboratory, the chronotype can be assessed accurately. Most of the individuals have a preference as intermediate with respect to these two extremes. The endogenous circadian clock of “early birds” tracks an earlier hour, as compared with that of “night owls,” with respect to both the social clock time and the individual wake-up time. These phenomena might explain the currently equivocal data on genetic associations with chronotype. Moreover, different genes for the chronotype are expressed in different generations. In such epigenetic landscape, a single genotype can thereby lead to different phenotypes. The continuum of chronotype diversity is largely mediated by genes, and the schedule of gene expression is fundamentally different, not just merely advanced or delayed, between the “early birds” and “night owls”. Further, there are no gender differences for the magnitude of the genetic and environmental effects, nor is the magnitude of these effects influenced by age. It is suggested that for a given chronotype, the genetic effects may not operate in an additive manner, but the underlying liability to chronotype was 38 % on average for the genetic factors due to dominance. On the basis of twin studies, it is estimated that the genetic effects contribute to the chronotype by about half and the other half is accounted by environmental factors. Of note, however, there was no influence of age on the endogenous circadian period. The estimates of the endogenous circadian period from 52 women and 105 men, healthy and aged 18–74 years, revealed that the period of the circadian rhythms of melatonin concentration and core body temperature was close to 24 h on average, but shorter in women (24 h 5 min ± 12 min) than that in men (24 h 11 min ± 12 min). The endogenous circadian period correlates with the subjective report of this behavioral trait, with the circadian phase, and with the wake-up time. These preferences form a continuum which has the anchorage ends of morning-oriented preference (“early birds”) and evening-oriented preference (“night owls”). Therefore, the behavioral trait of preference to schedule the daily activities for morning or evening hours can be estimated from the timing of sleep phase (bedtime to wake-up time), although the estimates need to be corrected for potential sleep debt. The endogenous circadian rhythms are normally aligned with the individual’s sleep-wake cycle. The individual’s sleep-wake (rest-activity) cycle is a product of the homeostatic sleep process and the circadian process that interact with each other but appears to be dictated mostly by the latter. Circadian clocks are endogenous pacemakers that evolve their properties, when subjected to selection, but have remained conservative during evolution. Thereafter, the period has lengthened, along with which circadian clocks have evolved a slowing-down mechanism and adopted periods of longer than 14 h up to the current one of about 24 h as the reference. Thus, circadian clocks met the night-day transitions with the approximate period of 14 h and were readily involved in protection from ultraviolet radiation. In the beginning of life, 3.5 to 3.9 billion years ago, the length of day was about 14 h and ultraviolet radiation was not filtered by the Earth’s atmosphere. These oscillations are generated and maintained by endogenous circadian clocks. Oscillations of endogenous biological processes that follow the approximate 24-h, that is circadian, rhythm exist in diverse organisms and arise as an adaptive mechanism to the Earth’s rotation producing the foreseeable transitions of night and day.
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