To clarify the demography of delayed sleep phase syndrome (DSPS), non-24-h sleep–wake syndrome, and irregular sleep–wake pattern in Japan, a cross-sectional nationwide epidemiological survey was conducted. 1525 adults (age: 15–59 years) were randomly sampled from telephone directories, and they received screening questions over the phone. Persons who were suspected of having the disorders were requested to fill out the second questionnaire, and asked to keep a sleep log for 4 weeks. Diagnoses were made according to the International Classification of Sleep Disorders criteria. As a result, the prevalence of DSPS was estimated to be 0.13%.