Blacks (African Americans) Have Shorter Free-Running Circadian Periods Than Whites (Caucasian Americans)

Abstract

The length of the free-running period (τ) affects how an animal re-entrains after phase shifts of the LD cycle. Those with shorter periods adapt faster to phase advances than those with longer periods, while those with longer periods adapt faster to phase delays than those with shorter periods. The free-running period of humans, measured in temporal isolation units and in forced desychrony protocols in which the day length is set beyond the range of entrainment, ranges from about 23.5 to 26 hours, depending on the individual and the experimental conditions (e.g., temporal isolation vs. forced desychrony). We studied 94 subjects free-running through an ultradian LD cycle, which was a forced desychrony with a day length of 4 h (2.5 h awake in dim light, ~ 35 lux, alternating with 1.5 h for sleep in darkness). Circadian phase assessments were conducted before (baseline) and after (final) three 24-h days of the ultradian LD cycle. During these assessments, saliva samples were collected every 30 min and subsequently analyzed for melatonin. The phase shift of the dim light melatonin onset (DLMO) from the baseline to the final phase assessment gave the free-running period. The mean ± SD period was 24.31 ± .23 h and ranged from 23.7 to 24.9 h. Black subjects had a significantly shorter free-running period than Whites (24.18 ± .23 h, N=20 vs. 24.37 ± .22 h, N=55). We had a greater proportion of women than men in our Black sample, so to check the τ difference we compared the Black women to the White women. Again, Black subjects had a significantly shorter free-running period (24.18 ± .23, N=17 vs. 24.41 ± .23, N=23). We did not find any sex differences in the free-running period. These findings give rise to several testable predictions: on average, Blacks should adapt quicker to eastward flights across time zones than Whites, whereas Whites should adjust quicker to westward flights than Blacks. Also, Blacks should have more difficulty adjusting to night shift work and day sleep, which requires a phase delay. On the other hand, Whites should be more likely to have trouble adapting to the early work and school schedules imposed by society. More research is needed to confirm these results and predictions.

Ethnic differences in sleep duration and morning–evening type in a population sample

“…From a biological perspective, tau is a factor that contributes to the extent that someone is a morning or an evening type: a shorter tau contributes to morningness and a longer tau contributes to eveningness. Blacks have been found to have shorter tau’s (and therefore potentially more likely to be morning type) than other races (Eastman et al, 2012; Smith et al, 2009). Data from this study are one of the first to translate these clinical findings to racial/ethnic differences in chronotype at the population level.…”

Case–Control Study of Breast Cancer in India: Role ofPERIOD3 Clock Gene Length Polymorphism and Chronotype

“…The National Center of Sleep Disorders Research has expressed a need for surrogate measures of sleep and circadian physiology that can be readily implemented in population-based studies of chronic disease (73). Chronotype is a simple, reliable and valid measure that may help elucidate the potential relationship between circadian rhythm disruption and BrCA risk in areas where recent modernization may facilitate increased BrCA risk, such as India, or in populations with racial cancer disparities and inherent differences in endogenous circadian timing (74,75). …”

Phase advancing human circadian rhythms with morning bright light, afternoon melatonin, and gradually shifted sleep: can we reduce morning bright-light duration?

“…Advancing the system (shifting it earlier) is more difficult and typically takes longer than delaying (shifting it later). This may be in part because most humans have an endogenous period that is slightly longer than 24 hours [30–34] which favors the ability to delay. Of note, however, mice also have more difficulty advancing despite their average free-running period being less than 24 hours [35].…”