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Researchers have identified inadequate sleep duration as one of the factors contributing to global obesity. The purpose of this study was to test a hypothesis deduced from a new sleep-duration-based evolutionary theory claiming that sleep extension in response to lengthening night duration in early fall evolved into a behavioral marker of an approaching winter; this adaptive trait was theorized to produce adiposity gain in White men in response to sleep extension. The hypothesis was that White Americans would show a greater increase in the age-adjusted fat mass index per unit of sleep duration compared to that of Black Americans. Data were part of the National Health and Nutrition Examination Survey (NHANES) study between 2005 and 2010. The multiple regression analysis did not support the study hypothesis. The results indicated that habitual sleep duration had no effect on the annual rate of adiposity gain in White men, while in Black men, longer sleep was associated with significantly higher annual rates of adiposity gain. Implications for social change include the case for population-specific antiobesity interventions in Black men, including closer monitoring of sleep duration in order to prevent adverse habitual sleep extension and to improve time budgeting for physical exercise.