Sleep deprivation and insomnia may be risk factors for obesity due to decreased energy expenditure and increased caloric intake. The relationship between sleep duration, insomnia and weight and fat mass was assessed in postmenopausal women (N=92,706) in the Women’s Health Initiative Observational Study from 1993-2005. Baseline sleep duration and insomnia status, and annual weight information were collected by questionnaire. Body mass index (BMI) was calculated annually from height and weight measurements. Fat mass percent was assessed at baseline and every 3 years by dual-energy x-ray absorptiometry. After multivariable adjustment, baseline cross-sectional analyses demonstrated a significant relationship between sleeping 6 hours (β = 0.19, 95% CI: 0.10, 0.29) and 8 hours (β = 0.16, 95% CI: 0.06, 0.26) but not ≤ 5 or ≥ 9 hours and BMI (reference = 7 hours). After multivariable adjustment, baseline insomnia was associated with significantly lower BMI (β = -0.31, 95% CI: -0.39, -0.23). No association was observed between baseline sleep duration, insomnia status and fat mass percent. Prospectively, baseline sleep duration was not significantly associated with weight change or change in fat mass percent. Similarly, baseline insomnia status was not associated with change in weight or change in fat mass percent over time. As the prevalence of obesity increases, it is necessary to identify modifiable risk factors. Our findings suggest that sleep duration is not a factor for weight gain in post-menopausal women.