The genes that regulate circadian rhythms have been genetically characterized in flies and mice. In the April issue of EMBO Reports, Ebisawa et al. describe a screen for genetic polymorphisms associated with human circadian rhythm disorders (EMBO Reports 2001, 2:342-346). They performed a PCR-based analysis of the human period3 gene (hPer3), a homolog of a Drosophila clock gene, and identified 20 sequence variations, of which six predicted amino acid changes. Ebisawa et al. defined four haplotypes for the hPer3 gene. Screening a group of 48 patients with delayed sleep phase syndrome (DSPS), they discovered that the H4 haplotype was associated with DSPS in approx 15% of cases. The authors speculate that a structural polymorphism may affect hPer3 phosphorylation, causing DSPS susceptibility.