Abstract: The present invention includes the disclosure of the hPER2 gene and a mutant of the hPER2 gene that participates in the human circadian biological clock. The product of the mutant hPER2 gene found in some familial advanced sleep phase syndrome patients is hypophosphorylated by casein kinase epsilon due to the serine-to-glycine mutation caused by the point mutation of the genomic sequence. Specifically, this serine-to-glycine mutation affects the casein kinase epsilon binding region of the hPER2 protein, thus blocking the phosphorylation cascade ordinarily caused by the binding of casein kinase epsilon to hPER2.