familial advanced sleep phase (fasp) is characterized as a short period (e.g. 23.3 vs 24.3hr for population) in humans. a mutation in hper2 decreases its phosphorylation by ck1d, which causes the phenotype seen in some fasp. the primary cause of these fasp is a mutation that changes amino acid 662 from serine to glycine (s662g) in per2. the s662g mutation makes per2 mutant protein a stronger repressor than normal per2, decreasing cellular per2 levels and therefore causing this form of fasp. the mutation also seems to cause an increase in the turnover rate of per2 in the nucleus.