Rs121908635

rs121908635, also known as Ser662Gly or S662G, is a SNP in the period circadian clock 2 PER2 gene.

This SNP is associated with the sleep disorder known as Familial Advanced Sleep Phase Syndrome (FASPS), a highly penetrant autosomal dominant trait.,

Affected individuals are extreme "morning larks"; they show advanced sleep-phase syndrome, awakening and going to bed exceptionally early (by ~4 hours) to maintain a normal quantity and quality of sleep.