This study of prostate cancer patients who underwent radiation therapy (RT) examined how total sleep time changed from the time of simulation to four months after the completion of RT and investigated whether specific variables predicted initial levels of sleep disturbance and characteristics of the trajectories of sleep disturbance. Eighty-two men completed a number of measures (i.e., wrist actigraphy, General Sleep Disturbance Scale, Center for Epidemiologic Studies-Depression scale, Spielberger State-Trait Anxiety Inventories, Brief Pain Inventory) over six months. Descriptive statistics and hierarchical linear modeling were used for data analysis. Large amounts of inter-individual variability were found in the trajectories of sleep disturbance. At baseline, sleep disturbance were associated with ethnicity, KPS score, total dose of RT, pain at baseline. The trajectories of sleep disturbance in this group of men with prostate cancer were associated with total dose of RT, pain at baseline, total sleep time at baseline, state anxiety at baseline. This study is the first using actigraphy to identify predictors of inter-individual variability in sleep disturbance in men with prostate cancer undergoing RT. Additional research is warranted to confirm this study's findings and to develops and test interventions in this vulnerable group of oncology patients.