Morning light exposure within the first hour of waking advances the circadian phase by activating ipRGCs and inducing Per1/Per2 expression in the SCN[[https://ouraring.com/blog/benefits-of-morning-sunlight/]][https://www.hubermanlab.com/newsletter/using-light-for-health]. Evening blue light around 480 nm delays the phase and suppresses melatonin[[https://www.sleepfoundation.org/bedroom-environment/blue-light]][[https://www.chronobiologyinmedicine.org/journal/view.php?number=167&viewtype=pubreader]]. Recent work shows that light signals reach peripheral clocks through ipRGC‑SCN‑sympathetic pathways[[https://doi.org/10.1073/pnas.1719548115]] and that exercise timing can further entrain muscle clocks via AMPK[[https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2025.1633835/full]]. However, SCN amplitude declines with age[[https://pmc.ncbi.nlm.nih.gov/articles/PMC4456861/]], potentially weakening the ipRGC‑SCN coupling.

Hypothesis: Chronic exposure to low‑intensity blue‑enriched indoor lighting (≈30 lux, 460‑480 nm) during the day reduces ipRGC sensitivity, thereby blunting the phase‑advancing effect of subsequent morning bright light (≥1000 lux). This attenuation is more pronounced in individuals over 60 years due to age‑related SCN amplitude loss and can be rescued by a brief N‑acetylcysteine (NAC) dose administered 30 minutes before morning light, which restores ipRGC signaling by lowering oxidative stress in retinal ganglion cells.

Testable predictions:

1. In a within‑subject crossover study, participants receiving two hours of daytime low‑level blue light will show a smaller morning light‑induced advance in dim‑light melatonin onset (DLMO) compared with a daytime dim‑light control condition (expected difference ≥30 min).
2. The DLMO attenuation will correlate with reduced ipRGC‑mediated pupillary constriction amplitude measured via infrared pupillometry.
3. Older adults (>60 yr) will exhibit a larger attenuation (≥45 min) than younger adults (<30 yr) under identical lighting conditions.
4. Pre‑treatment with 600 mg oral NAC before morning light will recover at least 80 % of the DLMO advance lost to daytime blue light exposure in both age groups.
5. Peripheral clock readouts (e.g., BMAL1 expression in vastus lateralis biopsies taken 4 h after morning light) will mirror the central phase shifts, showing improved synchrony only when NAC is administered.

Experimental design: Recruit 40 younger (20‑30 yr) and 40 older (60‑75 yr) healthy adults. Each participant completes four 3‑day sessions in random order: (A) daytime dim light (<10 lux) + morning bright light, (B) daytime low‑level blue light (30 lux, 470 nm) + morning bright light, (C) same as B with NAC pretreatment, (D) same as B with placebo. On day 3 of each session, collect hourly saliva for melatonin assay to compute DLMO, record pupil response to a blue light pulse, and obtain a muscle biopsy after morning light. Statistical analysis will use mixed‑effects models with session order as a covariate.

If confirmed, this hypothesis would reveal a previously underappreciated conflict between ubiquitous indoor lighting and circadian health, suggest a simple nutritional countermeasure, and refine light‑exercise timing guidelines for aging populations.