Few epidemiological studies have examined the potential protective effects of physical activity on insomnia. The authors thus evaluated the association between physical activity and insomnia in a large population-based study in Shizuoka, Japan. Individual data were obtained from participants in an ongoing cohort study. A total of 14,001 older residents who completed questionnaires were followed for 3 yr. Of these, 10,211 and 3,697 participants were eligible for the cross-sectional and longitudinal analyses, respectively. The authors obtained information about the frequency of physical activity and insomnia. Then, the adjusted odds ratios and 95% confidence intervals between physical activity and insomnia were estimated. Habitual physical activity was related to lower prevalence of insomnia. Frequent physical activity also reduced the incidence of insomnia, especially difficulty maintaining sleep. For elderly people with sufficient mobility and no preexisting disease, high-frequency physical activity (e.g., 5 or more days/wk) may help reduce insomnia.