A cross-sectional relation between short sleep and obesity has not been confirmed prospectively. The authors examined the relation between sleep duration and changes in body mass index and waist circumference using the Whitehall II Study, a prospective cohort of 10,308 white-collar British civil servants aged 35-55 years in 1985-1988. Data were gathered in 1997-1999 and 2003-2004. Sleep duration and other covariates were assessed. Changes in body mass index and waist circumference were assessed between the two phases. The incidence of obesity (body mass index: > or =30 kg/m(2)) was assessed among nonobese participants at baseline. In cross-sectional analyses (n = 5,021), there were significant, inverse associations (p < 0.001) between duration of sleep and both body mass index and waist circumference. Compared with 7 hours of sleep, a short duration of sleep (< or =5 hours) was associated with higher body mass index (beta = 0.82 units, 95% confidence interval (CI): 0.38, 1.26) and waist circumference (beta = 1.88 cm, 95% CI: 0.64, 3.12), as well as an increased risk of obesity (odds ratio(adjusted) = 1.65, 95% CI: 1.22, 2.24). In prospective analyses, a short duration of sleep was not associated with significant changes in body mass index (beta = -0.06, 95% CI: -0.26, 0.14) or waist circumference (beta = 0.44, 95% CI: -0.23, 1.12), nor with the incidence of obesity (odds ratio(adjusted) = 1.05, 95% CI: 0.60, 1.82). There is no temporal relation between short duration of sleep and future changes in measures of body weight and central adiposity.