Introduction Several studies from Western countries have reported that sleep duration is associated with weight gain or obesity. We examined the association between sleep duration and risk of weight gain or obesity in Japan.

Methods We combined data from two cohort studies. In 1994, we delivered a questionnaire to all individuals aged 40–79 years who lived in the catchment area of Ohsaki Public Health Center. Of 54 996 eligible individuals, 52 029 (95%) responded. In 2006, we delivered another questionnaire to all community-dwelling individuals aged over 40 years in Ohsaki City. Of 77 325 eligible individuals, 49 603 (64.2%) responded. The 9206 participants who answered both questionnaires were classified by self-reported sleep duration: ≤5 h (short sleep), 6 h, 7 h (reference), 8 h, and ≥9 h (long sleep). The main outcome measure was a weight gain of ≥10 kg or a body mass index (BMI) of ≥25 kg/m² (obesity) calculated from self-reported height and weight. We used logistic regression analyses to derive ORs and 95% CIs adjusted for sex, age, BMI, education, smoking, alcohol drinking, occupation, marital status, menopausal status, and caffeine beverage consumption.

Results We observed no association between sleep duration and risk of weight gain and obesity. Multivariate ORs for weight gain were 1.14 (95% CI 0.70 to 1.87) for short sleep and 1.16 (95% CI 0.90 to 1.51) for long sleep. Multivariate ORs for obesity were 0.98 (95% CI 0.62 to 1.55) for short sleep and 1.05 (95% CI 0.83 to 1.33) for long sleep.

Conclusion Sleep duration does not affect the risk of weight gain or obesity.