Hypothesis

Circadian regulation of the NAD+ salvage enzyme NAMPT creates a rhythmic NAD+ pool that gates SIRT1/3 activity and downstream autophagy flux. When feeding aligns with the active phase, NAD+ peaks during the early rest period, driving deacetylation of core clock components and autophagy regulators, thereby amplifying the geroprotective effects of time‑restricted feeding (TRF). Misaligned feeding blunts NAD+ oscillations, leading to chronic mTORC1 activation, insufficient autophagic clearance, and accelerated senescence.

Mechanistic Reasoning

• NAD+ rhythm generation: It's known that NAMPT transcription is directly driven by CLOCK:BMAL1 binding to E‑box promoters, producing a NAD+ surge ~4 h after the onset of the rest phase (2). This NAD+ peak activates SIRT1, which deacetylates BMAL1, reinforcing clock amplitude, and SIRT3, which deacetylates and activates mitochondrial enzymes involved in ROS detoxification (4).

• Autophagy coupling: We're proposing that SIRT1 deacetylates key autophagy proteins (e.g., ATG5, LC3) and inhibits mTORC1 via TSC2 activation, promoting autophagosome formation. SIRT3‑mediated mitochondrial deacetylation reduces ROS, limiting oxidative damage that would otherwise trigger the senescence‑associated secretory phenotype (SASP). Thus, a robust NAD+ rhythm couples cellular cleanup to the circadian cycle.

• Effect of misaligned feeding: Feeding during the rest phase elevates insulin and amino‑acid signals that activate mTORC1 irrespective of NAD+ levels, suppressing autophagy and causing NAD+ consumption through PARP‑1 activation due to increased DNA damage. We don't expect the NAD+ trough to merely lower energy; it actively weakens SIRT feedback, destabilising the clock and creating a feed‑forward loop of circadian decay and senescence.

Testable Predictions

1. Liver‑specific NAMPT knockout mice will show no lifespan extension from daytime‑aligned TRF, despite identical caloric intake, whereas wild‑type controls retain the ~35% benefit reported for timed CR (1).

2. Pharmacological NAD+ boosting (e.g., NMN) administered at the circadian time corresponding to the endogenous NAD+ peak will rescue the lifespan‑extending effect of TRF in NAMPT‑deficient mice, but the same supplementation given at the opposite phase will be ineffective.

3. In wild‑type mice, misaligned TRF (feeding during the rest phase) will lead to:

   • Elevated hepatic mTORC1 activity (phospho‑S6K) measured at ZT6,
   • Reduced LC3‑II/I ratio and increased p62 accumulation,
   • Increased senescence markers (p16^Ink4a, SASP cytokines) compared with aligned TRF.

These outcomes can be assessed by standard western blot, immunofluorescence, and qPCR assays, providing a clear falsifiable framework: if NAD+ rhythm is not required for the TRF longevity benefit, predictions 1 and 2 will fail.

Practical Implication

Optimizing NAD+ supplementation timing to coincide with the endogenous NAD+ surge may amplify the anti‑aging firewall of circadian alignment, offering a chronotherapeutic strategy that extends the geroprotective window beyond feeding schedules alone.