Recent progress in biological clock research has facilitated genetic analysis of 
circadian rhythm sleep disorders, such as delayed sleep phase syndrome (DSPS) 
and non-24-h sleep-wake syndrome (N-24). We analyzed the human period3 (hPer3) 
gene, one of the human homologs of the Drosophila clock-gene period (Per), as a 
possible candidate for rhythm disorder susceptibility. All of the coding exons 
in the hPer3 gene were screened for polymorphisms by a PCR-based strategy using 
genomic DNA samples from sleep disorder patients and control subjects. We 
identified six sequence variations with amino acid changes, of which five were 
common and predicted four haplotypes of the hPer3 gene. One of the haplotypes 
was significantly associated with DSPS (Bonferroni's corrected P = 0.037; odds 
ratio = 7.79; 95% CI 1.59-38.3) in our study population. Our results suggest 
that structural polymorphisms in the hPer3 gene may be implicated in the 
pathogenesis of DSPS.