Abstract
Successful adaptation to modern civilization requires the internal circadian clock to make large phase shifts in response to circumstances (e.g., jet travel and shift work) that were not encountered during most of our evolution. We found that the magnitude and direction of the circadian clock's phase shift after the light/dark and sleep/wake/meal schedule was phase-advanced (made earlier) by 9 hours differed in European-Americans compared to African-Americans. European-Americans had larger phase shifts, but were more likely to phase-delay after the 9-hour advance (to phase shift in the wrong direction). The magnitude and direction of the phase shift was related to the free-running circadian period, and European-Americans had a longer circadian period than African-Americans. Circadian period was related to the percent Sub-Saharan African and European ancestry from DNA samples. We speculate that a short circadian period was advantageous during our evolution in Africa and lengthened with northern migrations out of Africa. The differences in circadian rhythms remaining today are relevant for understanding and treating the modern circadian-rhythm-based disorders which are due to a misalignment between the internal circadian rhythms and the times for sleep, work, school and meals.
Highlights
Successful adaptation to modern civilization requires the internal circadian clock to make large phase shifts in response to circumstances that were not encountered during most of our evolution
We found that the magnitude and direction of the circadian clock’s phase shift after the light/dark and sleep/wake/meal schedule was phase-advanced by 9 hours differed in European-Americans compared to African-Americans
We found that African-Americans had shorter endogenous free-running circadian periods than European-Americans, and that circadian period (t) was correlated with the percent African and European genetic ancestry
Summary
Successful adaptation to modern civilization requires the internal circadian clock to make large phase shifts in response to circumstances (e.g., jet travel and shift work) that were not encountered during most of our evolution. The aims of the current prospective study (see the protocol in Fig. 1) were to examine phase shifts in response to a large (9-h), abrupt phase advance of 24-hour zeitgebers (24-hour time cues), and to measure the free-running circadian period in African-Americans compared to European-Americans with equal numbers in each group and equal numbers of men and women. We collected participant-reported race of parents and grandparents and buccal DNA samples to estimate genetic ancestry To our knowledge, this is the first study to compare the phase shifts of humans from different races, in this case AfricanAmericans and European-Americans, after a large abrupt phase shift of sleep as would be experienced after crossing many time zones or working night shifts. We do not know of any other studies of racial/ethnic differences in circadian rhythms, or any studies relating human circadian rhythms to genetic ancestry
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