Individuals with the 5-repeat PER3 VNTR sequence tend to have relatively penetrant phenotypic characteristics including delayed sleep phase syndrome, increased susceptibility to cognitive impairment after sleep deprivation, morning circadian preference and differences in the timing or levels of circadian hormone secretion (16,37,40), although some inconsistencies have been reported (21,39,63–65). The gene discussed is PER3; the disease is Cognitive impairment.