Hypothesis: Morning bright light exposure within 30 min of waking primes skeletal muscle AMPK activity, such that a subsequent bout of moderate-intensity exercise triggers a synergistic increase in HIF‑1α‑dependent PER2 phosphorylation, leading to a greater advance of the central circadian phase than either stimulus alone, particularly in individuals over 65 years old.

Mechanistic rationale: Light activation of melanopsin-expressing retinal ganglion cells elevates intracellular Ca2+ and stimulates CaMKKβ‑AMPK signaling in peripheral tissues【1】【2】. Exercise raises the AMP:ATP ratio, directly activating AMPK through LKB1. Convergent AMPK activation phosphorylates HIF‑1α at specific residues, stabilizing it and enhancing its transcriptional co‑activator function on the PER2 promoter, while also facilitating CK1δ/ε‑mediated PER2 phosphorylation that accelerates PER2 degradation and phase advancement【8】. In aging, reduced melanopsin sensitivity diminishes baseline light‑induced AMPK activation【7】; exercise can compensate, but only when preceded by light that “primes” the pathway.

Testable predictions:

1. In adults aged ≥ 65 years, a three‑day regimen of 10,000 lux light for 30 min at 07:30 followed by 30 min of brisk walking at 08:30 will produce a larger phase advance of dim‑light melatonin onset (DLMO) than light alone, exercise alone, or a dim‑light control condition.
2. The additive phase shift will be attenuated by pharmacological AMPK inhibition (Compound C) or HIF‑1α knock‑down in peripheral blood mononuclear cells, as evidenced by reduced p‑AMPK, p‑HIF‑1α, and p‑PER2 levels.
3. Younger adults (18‑35 y) will show no significant difference between the combined condition and the best single condition, reflecting intact SCN light sensitivity.

Falsifiability: If the combined light‑exercise condition fails to produce a statistically significant greater DLMO advance compared with the most effective single condition, or if AMPK/HIF‑1α inhibition does not diminish the observed molecular markers and phase shift, the hypothesis is falsified.

Experimental design: Randomized, crossover trial with 60 older participants. Each undergoes four 3‑day interventions separated by ≥1‑week washout: (A) bright light only, (B) exercise only, (C) light → exercise, (D) dim light + rest (control). Primary outcome: DLMO shift measured via saliva melatonin assays. Secondary outcomes: Western blot quantification of p‑AMPK (Thr172), p‑HIF‑1α (Ser551), and p‑PER2 (Ser453/Ser457) in PBMCs collected pre‑ and post‑intervention. Statistical analysis via repeated‑measures ANOVA with post‑hoc contrasts.

By linking circadian photic entrainment to exercise‑induced metabolic signaling via AMPK/HIF‑1α, this hypothesis extends the current focus on light‑only or exercise‑only strategies and proposes a personalized, time‑dependent combo that could mitigate age‑related circadian decline.