rs121908635

A SNP in the PER2 gene on chromosome 2. Its most notable genotype is C;T: Morning Lark (people waking up very early)

Gene
PER2
Location
Chr 2238,257,003 (GRCh38)
Highest magnitude
4.0Very important
Tested on
23andMe, AncestryDNA

Genotypes

Plus strand, as in 23andMe and AncestryDNA files.

  1. T;TGoodcommon in clinvarTypical · 0.0
  2. C;TMost notableNot ratedMorning Lark (people waking up very early)Very important · 4.0

SNPedia records rs121908635 on the minus strand: (A;A), (A;G). Magnitude rates how notable a genotype is from 0 to 10, not how risky it is.

Conditions, medicines and genes

Notes

From SNPedia

Show notes, about 120 words

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.

C;T

This variant is associated with people who have a sleep disorder known as Familial Advanced Sleep Phase sydrome (FASPS). This is associated with a 4-hour advance of their sleep, temperature, and melatonin rhythms, including going to bed and waking up that much earlier on average.

Source and license

Adapted from rs121908635 on SNPedia, using a snapshot taken October 16, 2026. SNPedia content is shared under CC BY-NC-SA 3.0 US, and this page is shared under the same license.

SNPedia is a community-edited wiki. Its entries can be incomplete, out of date or wrong, and nothing here is a diagnosis or medical advice. Magnitude is a community rating of how interesting a genotype is, not a risk score.