Circadian decline drives aging, but the exact pathway linking light input to muscle NAD+ remains unclear. We hypothesize that morning exposure to blue-enriched light activates retinal intrinsically photosensitive ganglion cells (ipRGCs), which increase sympathetic outflow to skeletal muscle. This sympathetic tone raises norepinephrine levels, stimulating β‑adrenergic receptors on myofibers and triggering a cAMP‑PKA‑CREB cascade that upregulates the NAD+ biosynthetic enzyme NAMPT. Elevated NAMPT boosts intracellular NAD+, enhancing SIRT1 activity and improving the deacetylation‑acetylation cycle of BMAL1, thereby reinforcing circadian amplitude. Stronger clock function then restores downstream pathways such as mTOR inhibition, FOXO activation, and mitochondrial biogenesis, collectively reducing oxidative stress, inflammation, and protein turnover defects that underlie sarcopenia.

This hypothesis is testable. First, expose young and aged mice to a defined morning blue‑light protocol (460 nm, 200 lux, 2 h) versus dim red‑light controls for four weeks. Measure muscle sympathetic nerve activity via norepinephrine microdialysis, NAMPT mRNA and protein levels, NAD+ concentrations, SIRT1 deacetylase activity, and BMAL1 acetylation status. Second, assess functional outcomes: grip strength, fiber cross‑sectional area, and markers of senescence (p16^INK4a^, SA‑β‑gal). Third, use pharmacological or genetic interventions to break the chain—β‑blocker propranolol, muscle‑specific NAMPT knockout, or ipRGC ablation (Opn4^−/−) — to determine whether each step is necessary for the light‑induced benefits. Finally, correlate changes in circadian amplitude (measured by PER2::LUC bioluminescence in explanted muscle) with the magnitude of sarcopenia mitigation.

If the hypothesis holds, morning blue light will raise sympathetic norepinephrine, increase NAMPT and NAD+, enhance SIRT1‑mediated BMAL1 regulation, and improve muscle mass and strength, whereas blocking any link will abolish these effects. Conversely, if light fails to alter sympathetic tone or NAMPT despite robust circadian entrainment via the SCN, the hypothesis will be falsified, prompting a search for alternative muscle‑specific light signaling routes. This work bridges environmental chronobiology, neuro‑muscular communication, and NAD+ metabolism, offering a concrete, non‑pharmacological strategy to preserve circadian fidelity and delay age‑related muscle decline.