A variable-number tandem repeat (VNTR) in the human clock gene PERIOD3 (PER3) has been suggested to correlate with a morning (lark) versus evening (owl) chronotype as well as with the circadian rhythm sleep disorder ?delayed sleep phase disorder?(DSPD).
For instance, a point mutation in a human clock gene (Per2) was shown to produce the rare advanced sleep phase syndrome, whereas a functional polymorphism in Per3 is associated with the more frequent delayed sleep phase syndrome.
Our results suggest that latitude has a role in the influence of hPer3 gene polymorphism on delayed sleep-phase syndrome and confirm previous data showing its association with morningness-eveningness tendencies.