This study determined the prevalence of and factors associated with insomnia in rotating-shift nurses. A two-stage, cross-sectional, hospital-based study was conducted in eastern Taiwan. Participants were randomly-sampled, rotating-shift nurses (n = 661), ranging in age from 21 to 62 years, with a mean age of 31.86 (standard deviation = 8.09). Insomnia disorder was identified using the Pittsburgh Sleep Quality Index. Nurses completed the self-reported Index, together with other questionnaires designed by the researchers for the purpose of the study. The prevalence of insomnia disorder was 59% (n = 390). Multivariate logistic regression analyses revealed that poor sleepers are more likely to have higher anxiety, feelings of depression, and a poor working atmosphere. Anxiety, depression, and working atmosphere are independent predictors of insomnia. These results suggest that it is crucial to implement a more appropriate shift system and to develop prevention programs for nurses with insomnia to improve their occupational health.