Study Objective: Actigraphy, a method of inferring sleep from the presence or absence of wrist movement, has been well validated against polysomnography in trials with people without insomnia. However, the small amount of literature on validation with insomniacs has revealed an actigraphy bias toward overscoring sleep. The current validation trial with insomniacs used the largest number of subjects to date in such research and attracted participants with diverse demographic characteristics. Design: People with insomnia slept 1 night in the laboratory while simultaneously being monitored by polysomnography, actigraphy (high-sensitivity algorithm of the Mini Mitter AW64 Actiwatch™), and morning sleep diary. Setting: Sleep disorders center. Participants: Participants were 57 volunteers from the community, 26 men and 31 women, ranging in age from 21 to 87 years. All participants satisfied conservative criteria for insomnia. The sample included subjects with primary insomnia, subjects with comorbid insomnia, and hypnotic users with current insomnia complaints. Interventions: N/A. Measurements and Results: Actigraphy was successfully validated on 4 measures of sleep pattern—number of awakenings, wake time after sleep onset, total sleep time, and sleep efficiency percentage—based on nonsignificant mean differences and significant correlation between actigraphy and polysomnography. Sleep-onset latency with actigraphy was not significantly different from polysomnography but was weakly correlated with polysomnography. Hypnotic use contributed to actigraphic overscoring of sleep. Conclusions: Actigraphy proved to be a satisfactory objective measure of sleep on 4 of 5 sleep parameters, but these results are specific to this particular instrument using this particular algorithm and should not be construed as a blanket endorsement of actigraphy for measuring insomnia.