OBJECTIVE: To assess how change in sleep duration is related to subsequent obesity. METHODS: In this 10-year follow-up, 4903 non-pregnant participants answered a questionnaire on sleeping habits, obesity, and lifestyle factors (questions identical to baseline questionnaire). Habitual normal sleepers were defined as sleeping 6-9 h/night at both baseline and follow-up, whereas women sleeping <6 h/night or ≥9 h/night at both occasions were defined as habitual short sleepers and habitual long sleepers, respectively. Logistic regression was used to analyze associations between changes in sleep duration, general obesity (body mass index ≥30 kg/m(2)), weight gain (≥10 kg) and also, central obesity (waist circumference ≥88 cm), and increase in waist circumference (≥10 cm) at follow-up. RESULTS: Among younger women (aged <40 years) both habitual short sleepers and habitual long sleepers had a higher prevalence of general (short: 31.3%, P < 0.0001; long: 38.1%, P = 0.01) and central obesity (short: 60.5%, P = 0.01; long: 82.4%, P = 0.01) compared with habitual normal sleepers (general obesity: 8.9%; central obesity: 35.9%) at follow-up. Younger women who were short sleepers at baseline but normal sleepers at the follow-up had a higher prevalence of both general (19.3%, P = 0.01) and central obesity (45.4%, P = 0.07) compared with habitual normal sleepers at follow-up. In adjusted analyses, both habitual short [adjusted odds ratio (aOR), 6.78; 95% confidence interval (CI), 2.71-17.0] and long (aOR, 4.64; 95% CI, 1.09-19.8) sleep durations were risk factors for general obesity in younger women. In younger women habitual long sleep duration was a risk factor also for central obesity (aOR, 6.05; 95% CI, 1.19-30.7) whereas habitual short sleep duration was not (aOR, 1.93; 95% CI, 0.87-4.81). Similar results were seen also for weight gain and increased waist circumference as dependent variables. In addition, decreased sleep duration from normal to short duration was a risk factor for both weight gain (aOR, 1.85; 95% CI, 1.14-3.02) and increased waist circumference (aOR, 1.84; 95% CI, 1.20-2.81). There were no associations between changes in sleep duration and any of the measures of obesity at the follow-up in women aged >40 years at baseline. CONCLUSION: In younger women, both habitual short and long sleep duration was a risk factor for obesity, whereas no such relationship was seen in older women.