Hypothesis

Circadian disruption drives inflammaging primarily by desynchronizing NAD+‑dependent SIRT1 activity from mitochondrial ROS production and lysosomal autophagy flux. Restoring the temporal NAD+ rhythm—through timed supplementation of NAD+ precursors—re‑synchronizes these pathways, thereby reducing oxidative damage, senescent cell accumulation, and chronic inflammation, and extending healthspan.

Mechanistic Rationale

NAD+ levels oscillate in a circadian manner, peaking during the active phase, and directly regulate SIRT1 deacetylase activity. SIRT1 targets both core clock components (e.g., BMAL1 deacetylation enhances DNA binding) and key effectors of mitochondrial quality control (e.g., deacetylation of SOD2 and PGC‑1α) and autophagy machinery (e.g., deacetylation of LC3 and ATG5). When NAD+ rhythms flatten with age, SIRT1 activity becomes constitutively low or mistimed, uncoupling:

• Mitochondrial antioxidant defenses from ROS peaks, leading to cumulative oxidative damage.
• Autophagosome formation from lysosomal acidification, causing incomplete cargo clearance and senescence‑associated secretory phenotype (SASP). Thus, loss of NAD+ rhythm creates a double hit: increased damage generation coupled with decreased damage removal, a core driver of inflammaging.

Testable Predictions

1. In aged mice, timed NAD+ precursor (nicotinamide riboside, NR) administration at the circadian peak (ZT4) will restore NAD+ amplitude >30% versus baseline, whereas constant NR dosing will not.
2. Restored NAD+ rhythm will increase SIRT1 deacetylation of BMAL1, SOD2, and LC3 specifically during the active phase, measured by immunoprecipitation‑Western blot.
3. Mitochondrial ROS (MitoSOX fluorescence) will show reduced peak-to-trough amplitude and lower overall levels only in the timed NR group.
4. Lysosomal cathepsin activity (Magic Red assay) will become phase‑aligned with autophagosome LC3‑II levels, reducing p62 accumulation and SASP markers (IL‑6, IL‑1β, SASP‑seq).
5. Consequently, senescent cell burden (p16^INK4a^ staining) will decrease by ≥25% and median lifespan will extend by ≥10% relative to controls. If timed NAD+ fails to improve any of these readouts, the hypothesis is falsified.

Experimental Design

• Animals: 20‑month‑old C57BL/6J mice, both sexes, n=15 per group.
• Groups: (1) Vehicle control, (2) Constant NR (300 mg/kg/day in drinking water), (3) Timed NR (same total dose delivered via osmotic pump programmed to release 70% of daily dose at ZT4 and 30% at ZT16).
• Duration: 6 months.
• Measurements:
  • Circadian NAD+ levels (LC‑MS/MS) sampled every 4h over 48h at midpoint and endpoint.
  • SIRT1 substrate acetylation (IP‑WB) at ZT4 and ZT16.
  • Mitochondrial ROS (MitoSOX) and membrane potential (TMRM) in isolated muscle and brain mitochondria.
  • Autophagic flux (LC3‑II/I ratio with/without bafilomycin A1) and lysosomal activity (LysoTracker, cathepsin B/L) in liver and hippocampus.
  • SASP cytokines (multiplex ELISA) and p16^INK4a^+ cell frequency (immunohistochemistry).
  • Frailty index, grip strength, and survival monitoring.
• Analysis: Two‑way ANOVA (treatment × time) for rhythmic data; post‑hoc tests for group comparisons; Kaplan‑Meier for survival.

Potential Outcomes and Interpretation

• Supportive outcome: Timed NR restores NAD+ oscillations, reinstates phase‑specific SIRT1 activity, lowers mitochondrial ROS, aligns autophagy with lysosomal degradation, reduces senescent cells and SASP, and extends median lifespan. This would confirm that circadian NAD+ rhythm is a linchpin linking metabolic state to damage control, positioning timed NAD+ boosting as a potent geroprotective strategy.
• Refutative outcome: Timed NR does not improve NAD+ amplitude, SIRT1 targeting, ROS/autophagy alignment, or health metrics compared to constant NR or vehicle. This would suggest that NAD+ rhythm alone is insufficient, or that downstream uncoupling (e.g., irreversible SIRT1 oxidation, lysosomal membrane damage) dominates inflammaging, redirecting focus to combined NAD+ and lysosomal rejuvenation approaches.

By directly testing whether re‑instating a single metabolic rhythm can re‑synchronize multiple arms of cellular maintenance, this hypothesis bridges circadian biology, redox metabolism, and autophagy, offering a clear, falsifiable path to a novel anti‑aging intervention.