Hypothesis

Evening supplementation with N‑acetylcysteine (NAC) reduces melatonin suppression caused by blue‑light exposure by scavenging mitochondrial reactive oxygen species (ROS) that amplify signaling in intrinsically photosensitive retinal ganglion cells (ipRGCs). This effect occurs without impairing the phase‑advancing benefits of morning blue‑enriched light.

Rationale

• Morning blue‑enriched light advances circadian phase through AMPK activation and HIF‑1α induction, stabilizing clock proteins and enhancing melatonin amplitude [[https://naturaled.com/lighting-and-health-trends-for-2026-and-beyond/]] [[https://www.sciencedaily.com/releases/2025/02/250225122145.htm]] [[https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2025.1633835/full]].
• Evening blue light (460‑480 nm) suppresses melatonin via ipRGCs, whose signaling strength is modulated by intracellular ROS that potentiate melanopsin‑driven calcium influx [[https://www.health.harvard.edu/healthy-aging-and-longevity/blue-light-has-a-dark-side]] [[https://www.chronobiologyinmedicine.org/m/journal/view.php?number=167]] [[https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2025.1633835/full]].
• NAC is a precursor for glutathione and directly scavenges mitochondrial ROS, reducing oxidative signaling without affecting photon capture by melanopsin [[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6052363/]].
• Genetic variants in PER3, CRY1, NPAS2, and BMAL1 alter individual sensitivity to blue light [[https://www.chronobiologyinmedicine.org/m/journal/view.php?number=167]]; ROS‑dependent signaling may be a common downstream effector linking genotype to phenotype.

Predictions

1. In healthy adults, 600 mg NAC taken 30 minutes before a 2‑hour evening blue‑light exposure (480 nm, 30 lux) will produce a smaller melatonin suppression (area under the curve) than placebo.
2. The magnitude of NAC‑mediated attenuation will correlate with baseline mitochondrial ROS levels measured in peripheral blood mononuclear cells.
3. Morning NAC administration (same dose, 30 minutes before 30 minutes of blue‑enriched light) will not diminish the phase‑advancing effect assessed by dim‑light melatonin onset (DLMO) shift.
4. Individuals carrying PER3^5/5 or CRY1 rs2287161 risk alleles will show greater NAC‑responsive melatonin protection, reflecting higher ROS‑dependent signaling.

Experimental Design

• Participants: 120 adults aged 20‑35, stratified by genotype (PER3, CRY1, NPAS2, BMAL1) and baseline melatonin amplitude.
• Design: Double‑blind, crossover, four‑condition protocol (evening blue light + NAC, evening blue light + placebo, morning blue light + NAC, morning blue light + placebo) with ≥1‑week washout.
• Procedures:
  • Salivary melatonin sampled every 20 minutes from 2 hours before to 4 hours after light exposure.
  • DLMO calculated via logistic fit.
  • Mitochondrial ROS assessed via MitoSOX fluorescence in isolated PBMCs at baseline.
  • Subjective sleep quality and next‑day psychomotor vigilance test (PVT) recorded.
• Analysis: Mixed‑effects models with fixed effects for treatment, genotype, and time; random intercepts for participants. Interaction terms test whether NAC effect differs by genotype.

Potential Outcomes

If NAC selectively blunts evening melatonin suppression without affecting morning phase advances, it would reveal a modifiable redox node downstream of ipRGC activation that gates circadian disruption. This would support personalized interventions: individuals with high ROS‑sensitive genotypes could use timed antioxidant pretreatment to mitigate screen‑induced sleep loss while preserving the therapeutic value of morning blue light. Conversely, a null result would suggest that melatonin suppression is driven primarily by photon capture rather than ROS amplification, refocusing mechanistic inquiry on melanopsin kinetics or downstream transcriptional cofactors.