Study Objectives: To investigate the link between extreme diurnal preference, delayed sleep phase syndrome, and a length polymorphism in Per3. Design: Subjects were genotyped using polymerase chain reaction. Patients or Participants: Subjects with defined diurnal preference as determined by the Horne-Östberg questionnaire and patients with delayed sleep phase syndrome. Measurements and Results: The Per3 polymorphism correlated significantly with extreme diurnal preference, the longer allele associating with morningness and the shorter allele with eveningness. The shorter allele was strongly associated with the delayed sleep phase syndrome patients, 75% of whom were homozygous. Conclusion: The length of the Per3 repeat region identifies a potential genetic marker for extreme diurnal preference.