To determine the prevalence of the delayed sleep phase syndrome (DSPS) and the contrasting advanced sleep phase syndrome (ASPS), a cross-sectional nationwide epidemiological study was performed in Norway. Screening questionnaires were sent to a random sample of 10,000 adult individuals (18-67 y), of both sexes, taken from the National register of Norway. The response rate was 77%. Diagnoses of DSPS and ASPS were based on International Classification of Sleep Disorders (ICSD) criteria. All individuals suspected of having DSPS or ASPS were requested to fill out a second questionnaire, and a sleep log for four weeks. Subjects for whom the suspicion of DSPS or ASPS could be upheld were contacted by telephone for a final confirmation. Of the 129 possible DSPS cases identified from the screening questionnaires, 17 (9 f; 8 m) remained with the confirmed diagnosis of DSPS. The prevalence was calculated to be 0.17% (95% Confidence Intervals: 0.10-0.28). Thirteen individuals had a mild to moderate DSPS and four had a severe DSPS. The mean age of onset was 15.4 y, and mean duration was 19.2 y. There was no significant correlation between prevalence and age. A sleep phase delay (MSPD) induced by social/environmental or psychological factors was found in 55 subjects (prevalence = 0.72%). Using strict ICSD criteria, no case of ASPS was detected, confirming earlier assumptions of the extreme rarity of this condition.