rs55694368 — PER2 PER2 regulatory variant

PER2 Upstream Variant — A Regulatory Switch for Your Sleep Timing

About 121 kilobases upstream of the PER2 gene¹¹ PER2 gene
Period Circadian Regulator 2: one of the core negative-feedback proteins in the mammalian circadian clock. PER2 protein accumulates during the day, enters the cell nucleus to inhibit its own transcription, then gets degraded — resetting the clock for the next 24-hour cycle sits a regulatory region that acts as a long-range tuner of PER2 expression. The rs55694368 variant in this region was first identified in a 2016 genome-wide association study of nearly 90,000 people and independently confirmed in a 2019 meta-analysis of 697,828 individuals — one of the largest chronotype studies ever conducted. Carriers of the T allele (about 13% of people of European ancestry) show a measurable shift in their biological clock toward eveningness.

The Mechanism

PER2 expression is tightly controlled not just by its promoter but by distal enhancer elements²² enhancer elements
Stretches of DNA, often tens to hundreds of kilobases from a gene, that bind transcription factors and loop back to the promoter to boost or tune gene expression. They are especially important for genes like PER2 that must be precisely timed that bind CLOCK/BMAL1³³ CLOCK/BMAL1
The master activator complex of the mammalian circadian clock. CLOCK and BMAL1 proteins form a dimer that drives transcription of PER2 and other clock genes; PER2 protein in turn accumulates and inhibits CLOCK/BMAL1 — completing the core feedback loop and other transcription factors. rs55694368 sits in one such upstream regulatory region. The T allele is thought to reduce the efficiency of this enhancer, producing slightly less PER2 protein per cell cycle. Because PER2 is a transcriptional repressor that drives its own oscillation, reduced levels slow the feedback loop slightly, delaying the phase of the clock — shifting the entire sleep-wake cycle toward later timing. This is a distinct regulatory haplotype from the PER2 5'UTR variant (rs2304672) and the coding variant rs35333999, meaning rs55694368 tags an independent route to the same outcome.

The Evidence

The initial discovery came from Hu et al.⁴⁴ Hu et al.
Hu Y et al. GWAS of 89,283 individuals identifies genetic variants associated with self-reporting of being a morning person. Nat Commun, 2016, a genome-wide association study of 89,283 individuals from the 23andMe cohort. The study identified seven loci near established circadian genes, including rs55694368 at PER2, reaching genome-wide significance (P=2.6×10⁻⁹). The G allele carried OR≈1.16 for self-reported morningness, meaning T carriers have approximately 14% lower odds of being a morning person per copy of T (OR≈0.86 for morningness per T allele copy).

Replication and extension came from the landmark Jones et al. chronotype GWAS⁵⁵ Jones et al. chronotype GWAS
Jones SE et al. Genome-wide association analyses of chronotype in 697,828 individuals provides insights into circadian rhythms. Nat Commun, 2019, which expanded the known genetic architecture of chronotype from 24 to 351 loci in a meta-analysis of UK Biobank and 23andMe data. The PER2 region was confirmed among the circadian-gene-enriched set. Mendelian randomization in that study found that genetically predicted later chronotype causally associates with poorer mental health outcomes, independent of sleep duration — adding clinical weight to this locus beyond its effect on sleep timing alone.

The effect size at rs55694368 is modest in isolation — as expected for a common regulatory variant influencing a complex trait — but it tags a distinct regulatory haplotype from the other two PER2 variants in the GeneOps database (rs2304672 and rs35333999), providing independent information about your circadian regulation.

Practical Implications

Chronotype is substantially genetic, and the T allele at rs55694368 is one piece of that genetic architecture. Carriers of one or two T alleles have a slightly stronger biological tendency toward later sleep timing. For most people in standard day-shift jobs, this manifests as needing slightly more effort to maintain early schedules. For T allele carriers in shift work⁶⁶ shift work
Work schedules that rotate across evenings, nights, and weekends, forcing the body clock to repeatedly misalign with work and social timing. Epidemiological studies link shift work to elevated rates of metabolic syndrome, cardiovascular disease, depression, and cancer or trans-meridian travel, the mismatch between biology and schedule can be more pronounced. The practical tools for managing chronotype — strategic light exposure, light-avoidance at night, and consistent anchoring of the sleep-wake cycle — are particularly valuable when the genetic tendency runs counter to social demands.

Interactions

rs55694368 tags a regulatory haplotype that is independent of, and potentially additive with, the PER2 5'UTR variant rs2304672 (associated with morning preference, opposite direction to rs55694368-T) and the coding variant rs35333999 (V903I, also eveningness-associated). A carrier of both rs55694368-TT and rs35333999-TT would carry two independent genetic pushes toward eveningness within the same gene. The CLOCK activator rs1801260 sits on the opposite side of the feedback loop: the G allele (eveningness-associated) and rs55694368-T may compound toward later sleep timing through distinct mechanisms — rs1801260 reducing CLOCK transcriptional activity and rs55694368 reducing PER2 enhancer response. Combined effects across these PER2 and CLOCK loci have not been formally tested but represent a plausible interaction for carriers of multiple evening-preference alleles.