Hypothesis

PER3 multi‑SNP haplotypes modulate the sensitivity of the hypothalamic‑pituitary‑gonadal (HPG) axis to photoperiodic light, such that evening‑aligned haplotypes amplify testosterone decline under short‑day (winter‑like) exposure, while morning‑aligned haplotypes blunt this effect. The resulting testosterone fluctuation then gates prefrontal cortical dopamine signaling, determining whether cognitive performance follows sleep timing or becomes decoupled.

Mechanistic Rationale

PER3 functions as a transcriptional repressor within the core circadian clock, influencing the amplitude of BMAL1‑CLOCK driven PER2 expression. Evening haplotypes reduce repressor strength, lengthening the intrinsic period and delaying melatonin onset. Under artificial short‑day photoperiods (≤8 h light), the delayed phase leads to a larger misalignment between endogenous circadian time and external light‑dark cues, elevating sympathetic tone and suppressing Leydig cell LH responsiveness. This yields a steeper drop in serum testosterone compared with morning haplotypes, which maintain tighter phase locking and preserve HPG output.

Lower testosterone reduces dopaminergic tone in the medial prefrontal cortex (mPFC) via androgen‑dependent up‑regulation of tyrosine hydroxylase. Consequently, evening haplotype carriers experience a double hit: circadian misalignment‑driven sleep loss plus testosterone‑dependent dopaminergic deficit, impairing attentional control and reaction time. Morning haplotype carriers, despite similar sleep loss, retain sufficient testosterone‑driven dopamine to sustain cognition.

Testable Predictions

1. In a within‑subject crossover study, participants stratified by PER3 haplotype (evening vs morning) will show a greater percent change in morning testosterone after 3 days of 8 h light/16 h dark versus 12 h light/12 h dark, with evening haplotypes exhibiting ≥20 % larger decline (p < 0.05).
2. The testosterone change will mediate the effect of photoperiod on a psychomotor vigilance task (PVT) score, such that mediation analysis reveals a significant indirect effect for evening haplotypes only (bootstrapped 95 % CI not crossing zero).
3. Pharmacological rescue with transdermal testosterone will normalize PVT performance in evening haplotype participants under short‑day conditions, eliminating the genotype‑by‑photoperiod interaction.

Experimental Design

• Recruit 120 healthy men aged 21‑35, genotyped for PER3 rs57875989 and rs228697 (defining evening/morning haplotypes).
• Randomize to two photoperiod conditions in counter‑balanced order, each lasting 5 days with a 2‑day washout.
• Collect saliva testosterone at 08:00 and 20:00 daily, actigraphy‑derived sleep parameters, and PVT after each night.
• Use mixed‑effects models with fixed effects for haplotype, photoperiod, day, and their interactions; random intercepts for subject.
• Conduct mediation analysis (testosterone as mediator) separately for each haplotype.

Potential Confounds

Seasonal melatonin variations, baseline testosterone differences, and non‑photic stressors will be controlled by conducting the study indoors with fixed temperature, providing standardized meals, and measuring cortisol.

Implications

If confirmed, the model would reveal a genotype‑specific endocrine pathway by which light environments sculpt cognition, informing personalized chronotherapy and shift‑work scheduling.