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 light-dark (LD) cycle. Those with shorter periods adapt faster to phase advances than those with longer periods, whereas 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, varies from about 23.5 to 26 h, 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 baseline to 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 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. (Author correspondence: [email protected])

Keywords:

• Circadian clock
• Circadian rhythms
• Forced desynchrony
• Human
• Intrinsic period
• Melatonin
• Race
• Tau

ACKNOWLEDGMENTS

We thank the following people for their help with data collection: Daniel Alderson, Elisabeth Beam, Helen Burgess, Jillian Canton, Stephanie Crowley, Erin Cullnan, Rose Diskin, Sarah Garcia, Clifford Gazda, Heather Gunn, Cynthia Hiltz, Heather Holly, Clara Lee, Carlo Legasto, Vanessa Meyer, Jacqueline Munoz, Meredith Rathert, Yelizaveta Sorokin, Jessica Stroup, Christina Suh, Christine Tseng, and Nicole Woodrick.

Declaration of Interest: This work was supported by grants R01 HL086934 and R01 NR07677 to C.I.E. from the National Institutes of Health. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Heart, Lung, and Blood Institute, the National Institute of Nursing Research, or the National Institutes of Health. The National Heart Lung and Blood Institute, the National Institute of Nursing Research, and the National Institutes of Health had no involvement in designing the study, data collection, data analysis, and interpretation, writing of the manuscript, nor in the decision to submit the manuscript for publication.

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.