A twin study of sleep duration and body mass index.

The advent of artificial lighting, shift work, television, the internet, and a 24-hour economy have all curtailed sleep times. As a result, sleep deprivation has reached epidemic proportions in our society, with ∼25% of the population regularly obtaining insufficient sleep to maintain normal alertness.1,2 Although the optimal amount of sleep for humans is unknown, we now sleep 1.5 h/night less than we did in 1910.3 In the 2002 National Sleep Foundation Sleep in America Poll, 39% of adults reported sleeping < 7 h/night on weeknights.4 Meanwhile, human sleep need remains unchanged. As sleep duration has dropped, rates of obesity have increased, with the most recent National Health and Nutrition Examination Survey reporting that 33.3% of adult men and 35.3% of adult women are obese.5 Research suggests chronically reduced sleep times are associated with obesity.6–11 Experimental studies in humans show that sleep curtailment influences the neuroendocrine control of appetite in healthy individuals.12 Population-based studies demonstrate a significant U-shaped non-linear relationship between nightly sleep duration and body mass index (BMI).13,14 Compared to those sleeping 7–8 h/night, individuals sleeping ≤ 6 h are at greater risk of being obese.14 Prospective family and cohort studies have found short sleep duration is associated with the development of obesity over time.15–17 Although the relationship between sleep duration and BMI may be caused by environmental influences such as voluntary sleep restriction, many measures of sleep are heritable, raising the possibility that genetic factors are central to this association.18–23 To date, no studies have accounted for shared genetic and environmental factors when considering the relationship between sleep duration and BMI. BRIEF SUMMARY Current Knowledge/Study Rationale: Twin study methodologies were used to determine the relative importance of genetic and environmental contributions to the association between sleep duration and body mass index (BMI). Study Impact: An association was observed between short sleep duration and elevated BMI in a community based sample of US twins after accounting for familial factors such as genetics and shared environment. This supports the environmental hypothesis that voluntary changes to habitual sleep duration influences BMI independent of familial factors. Twins are identical in age and, if reared together, are typically well-matched for shared family background and numerous childhood/adolescent exposures. Because twins tend to be quite similar on variables that differ among unrelated individuals, studying them allows detection of subtle environmental effects, such as the effect of chronic partial sleep restriction on body weight. We therefore conducted a twin study to account for the potential shared genetic and environmental contributions to habitual sleep duration and obesity. Our goals were to: (1) determine if sleep duration was associated with BMI in our twin sample and (2) examine if the association between sleep duration and BMI was confounded by familial factors such as genetics and shared environment. If we observed an association following adjustment for between-twin pair effects, this finding would support the environmental hypothesis that voluntary changes to habitual sleep duration influences BMI independent of familial factors.