Obesity Epidemiology

332 EPIDEMIOLOGIC STUDIES OF DETERMINANTS OF OBESITYand obesity was found to weaken with increasing age. The ORs were 7.4, 8.1, 4.7, and 1.1 atages 27, 29, 34, and 40. The rate of change of BMI had a monotonic relationship with sleepduration averaged across the 13 years. Those sleeping less than 5 hours gained weight at arate of nearly 0.4 kg/m 2 /year while those sleeping more than 9 hours lost weight. Interestingly,sleep duration appeared to be more strongly associated with previous weight thanconcurrent or future weight. For example at age 29, the ORs for less than 6 hours of sleepwas 11.8 with obesity at age 27, 8.1 with obesity at age 29, and 4.6 with obesity at age 34.Gangwisch et al. 59 analyzed data from 9588 participants of the National Health andNutrition Examination Survey (NHANES) study. Heights and weights were measured atbaseline but at subsequent timepoints, self-reported weight was utilized. Sleep durationwas obtained from a question asking about only nocturnal sleep. At baseline, a U-shapedassociation was found with minimum obesity risk in 7-hour sleepers. The strength of thesleep-obesity relationship waned with age such that the ORs for obesity in those sleeping4 hours or fewer relative to 7-hour sleepers were 3.21, 1.81, and 1.71 among those aged 32to 49, 50 to 67, and 68 to 86. Because the strongest cross-sectional association betweensleep and weight was found in the youngest tertile, longitudinal analysis was only performedin this subgroup. Over 9 years, a linear relationship between sleep and weightgain was found with those sleeping 4 hours or fewer gaining 1.46 kg/m 2 and those sleeping10 hours or more gaining only 0.08 kg/m 2 . After adjusting for potential confounderssuch as alcohol and tobacco habits, sleep-related complaints, depression, and physicalactivity, mean BMI was found to increase only 0.05 kg/m 2 for every hour decrease insleep duration. This difference was not statistically significant.Patel et al. 14 followed 68,183 American women in the Nurses Health Study aged 45-65for up to 16 years. Questionnaire response to habitual sleep duration over a 24-hourperiod was obtained at baseline and self-reported weights were obtained at baseline andevery 2 years. Cross-sectionally, a U-shaped association was found with minimum weightin those sleeping 7 hours. Adjusting for differences in baseline weight, weight gain was1.14 and 0.71 kg greater over the 16 years among 5- and 6-hour sleepers, respectivelycompared to 7-hour sleepers. No significant difference was observed among those sleeping7 hours or more. The hazard ratios for developing obesity and a 15-kg weight gainwere 1.15 and 1.28, respectively, in 5-hour sleepers.SummaryOverall, research in adults suggest short sleepers are heavier than those who sleep 7-8 hourswith supportive findings from the majority of cross-sectional analyses as well as all threeprospective studies. Results regarding a positive association between obesity and longersleep durations are more mixed but this is in part due to the fact that many studiesdid not specifically separate long sleepers from normal sleepers. Interestingly, both theNHANES and Nurses Health Study found a positive association between long sleep andobesity assessed simultaneously but no association with future obesity, suggesting thecross-sectional relationship might be due to reverse causation or residual confounding. 14,59Potential mechanisms for an association between long sleep duration and obesity, whethercausal or not, include depression, low socioeconomic status, and societal isolation. 13 Ifthe cross-sectional relationship between sleep and weight truly is U-shaped, the effect ofshort sleep would be underestimated in studies that conducted linear regression analysessince this forces a linear relationship between the two variables. This could explain thenegative findings in two studies. 54,55Several groups have explored whether the sleep-weight association differs acrossimportant subgroups. Two studies have suggested the effect of reduced sleep on weight