['To investigate the link between extreme diurnal preference, delayed sleep phase syndrome, and a length polymorphism in Per3.', 'Subjects were genotyped using polymerase chain reaction.', 'Subjects with defined diurnal preference as determined by the Horne-Ostberg questionnaire and patients with delayed sleep phase syndrome.', '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.'] Is a length polymorphism in the circadian clock gene Per3 linked to delayed sleep phase syndrome and extreme diurnal preference?