rs35833281 — HCRTR2

HCRTR2 — Your Wake-Drive Receptor and Sleep Timing

The orexin system is the brain's primary arousal engine. Two neuropeptides — orexin A and orexin B (also called hypocretin-1 and hypocretin-2) — are produced in a small cluster of neurons in the lateral hypothalamus and project widely across the brain. They act on two receptors: HCRTR1 (orexin receptor 1), which responds preferentially to orexin A, and HCRTR2 (orexin receptor 2, encoded by this gene), which binds both orexins with roughly equal affinity. HCRTR2 is expressed in the tuberomammillary nucleus, locus coeruleus, and dorsal raphe¹¹ tuberomammillary nucleus, locus coeruleus, and dorsal raphe
key brainstem and hypothalamic nuclei that drive wakefulness through histamine, norepinephrine, and serotonin release — making it a central hub of the wake-promoting circuit.

The biological importance of HCRTR2 for sleep-wake timing is underscored by animal genetics: narcolepsy in Doberman Pinschers was traced to a loss-of-function mutation in the HCRTR2 gene, and HCRTR2-knockout mice show disrupted sleep architecture with frequent wake-to-sleep transitions during the active period. In humans, orexin neuron loss (as in narcolepsy type 1) causes uncontrolled sleepiness and cataplexy, confirming the orexin-HCRTR2 axis as the neurological brake on inappropriate sleep onset during daytime.

rs35833281 is an intronic variant within HCRTR2 that has been identified as a genome-wide significant chronotype signal in multiple large GWAS. The C allele at this position is the morningness allele.

The Mechanism

rs35833281 sits within intron sequence of HCRTR2²² intron sequence of HCRTR2
intronic variants can influence gene expression through effects on splicing efficiency, transcription factor binding, or chromatin accessibility, and is in linkage disequilibrium with nearby functional variants in the HCRTR2 locus — including what Jones et al. 2019 describe as a fine-mapped missense variant in HCRTR2. Whether rs35833281 is itself causal or a tag SNP for a nearby functional change remains to be determined by fine-mapping studies.

The downstream biology, however, is clear: variation in HCRTR2 function modulates the strength or timing of the wake-drive signal mediated by the orexin system. A stronger or earlier orexin signal biases the circadian output toward earlier activity onset, producing the morningness phenotype. The additive nature of the chronotype GWAS signal is consistent with orexin system variants acting as quantitative modulators of arousal timing rather than on/off switches.

The Evidence

The strongest evidence comes from Jones et al. 2019³³ Jones et al. 2019
Genome-wide association analyses of chronotype in 697,828 individuals provides insights into circadian rhythms. Nature Communications, which identified 351 loci associated with morning preference. At rs35833281, the C allele reached genome-wide significance (p = 2×10⁻³¹) with OR ~1.05 for being a morning person — a small but highly replicable per-allele effect. The HCRTR2 locus was among those characterised as harbouring a fine-mapped missense variant, suggesting biological plausibility beyond statistical association. Activity-monitor data in 85,760 participants confirmed that individuals carrying the most morningness alleles across all loci sleep approximately 25 minutes earlier than those carrying the fewest. Mendelian randomisation in this same dataset demonstrated that morning preference causally lowers schizophrenia risk (OR 0.89, p = 0.004) and increases subjective well-being (0.04 SD, p = 5×10⁻⁵).

An earlier Jones et al. 2016⁴⁴ Jones et al. 2016
Genome-Wide Association Analyses in 128,266 Individuals Identifies New Morningness and Sleep Duration Loci. PLoS Genetics independently implicated the HCRTR2 region in chronotype, providing cross-cohort replication across UK Biobank and 23andMe datasets.

Lane et al. 2017⁵⁵ Lane et al. 2017
Genome-wide association analyses of sleep disturbance traits. Nature Genetics further identified a locus near HCRTR2 in a composite sleep trait GWAS (combining chronotype, insomnia symptoms, and daytime sleepiness in 112,586 individuals), suggesting the HCRTR2 signal extends across multiple dimensions of sleep-wake behaviour, not chronotype alone.

Practical Actions

The C allele at rs35833281 is a morningness allele — each copy nudges the biological clock slightly earlier. For CC homozygotes, both copies reinforce an early-rising tendency; for CG heterozygotes, the effect is present but attenuated. GG individuals carry neither copy and are somewhat more likely to be evening-types or have neutral chronotype.

The orexin system is also a target of pharmacological intervention. Dual orexin receptor antagonists (DORAs) such as suvorexant and lemborexant block HCRTR1 and HCRTR2 to promote sleep onset — a mechanism directly relevant to this genetic locus. Strong morning types with CC genotype who use sleep aids should be aware that DORAs target the same pathway their genotype influences. Evening types (GG) with insomnia or circadian misalignment may particularly benefit from chronotype-informed light-timing strategies.

Interactions

The orexin system interacts with the core circadian clock. Variants in CLOCK, BMAL1, PER2, and CRY1 affect the timing of the molecular clock in each cell; orexin receptor variants modulate how strongly the wake-promoting signal overrides the circadian sleep pressure at any given time. In principle, combinations of strong-clock variants (e.g., PER2 advanced sleep phase variants) with strong-orexin variants (CC at rs35833281) would compound morningness; weak-clock + GG combinations would compound eveningness. These interaction effects have not been formally studied in humans but are biologically plausible.

The GWAS Catalog lists rs35833281 in proximity to HCRTR2 at the same locus as rs2271933, a known HCRTR2 variant also studied in narcolepsy and cluster headache literature. Whether rs35833281 and rs2271933 tag the same functional variant or independent effects within HCRTR2 requires further fine-mapping.