the per3 “length” polymorphism in the 54-bp repeat sequence in exon 18 (genbank accession no. ab047686) is a structural polymorphism due to an insertion or deletion of 18 amino acids in a region encoding a putative phosphorylation domain. the polymorphism has been associated with diurnal preference and delayed sleep phase syndrome. a longer allele polymorphism is associated with “morningness” and the short allele with “eveningness.” the short allele is also associated with delayed sleep phase syndrome. the length polymorphism has also been shown to inhibit adipogenesis and per3 knockout mice were shown to have increased adipose tissue and decreased muscle tissue compared to wild type. additionally, the presence of the length polymorphism has also been shown to be associated with type 2 diabetes mellitus (t2dm) patients as compared to non-diabetic control patients. the per3-p415a/h417r polymorphism has been linked to familial advanced sleep phase syndrome in humans, as well as to seasonal affective disorder, though when knocked in to mice, the polymorphism causes a delayed sleep phase.