Hypothesis

Boosting SIRT1 activity specifically during the night phase synergizes with circadian‑aligned time‑restricted feeding (iTRF) to produce a supra‑additive increase in autophagic flux, leading to greater lifespan extension than iTRF alone.

Mechanistic Rationale

Circadian gating of autophagy genes such as Atg1 and Atg8a peaks at night, aligning autophagosome formation with the fasting windowiTRF study. SIRT1 deacetylates core clock components BMAL1 and CLOCK, reinforcing circadian amplitude and directly activating autophagy transcription programsSIRT1‑clock link. Age‑related decline in SIRT1 blunts this coupling, causing mistimed autophagy despite intact fasting. Pharmacologic or genetic SIRT1 upregulation at night would therefore:

• Enhance BMAL1/CLOCK‑driven transcription of autophagy genes beyond the baseline circadian peak.
• Increase lysosomal biogenesis via TFEB deacetylation, improving autophagic flux completion.
• Reduce inflammasome signaling, limiting secondary damage that impairs autophagosome‑lysosome fusion. Thus, when SIRT1 is elevated only during the night, the temporal precision of autophagy is sharpened, converting a permissive fasting signal into a maximally effective clearance wave.

Testable Predictions

1. Mice receiving a SIRT1‑activating compound (e.g., SRT2104) administered exclusively at zeitgeber time 12 (ZT12, onset of dark) will show higher LC3‑II turnover and p62 degradation during the fasting phase than mice receiving the same total dose split across day and night.
2. Night‑restricted SIRT1 activation will extend median lifespan by >15 % over iTRF alone, while day‑restricted activation will not significantly differ from iTRF controls.
3. Genetic ablation of Atg7 in hepatocytes will abolish the lifespan benefit of night‑specific SIRT1 upregulation, confirming autophagy dependence.
4. Lysosomal acidification markers (LAMP1‑LysoTracker colocalization) will be elevated only in the night‑SIRT1 group, indicating enhanced flux completion.

Experimental Design

• Groups (n=30 per group, male C57BL/6J): (1) ad libitum control, (2) iTRF 16:8 (fast ZT0–ZT8), (3) iTRF + night‑SIRT1 activator (ZT12–ZT24), (4) iTRF + day‑SIRT1 activator (ZT0–ZT12), (5) iTRF + night‑SIRT1 activator + liver‑specific Atg7 KO.
• Interventions: SIRT1 activator delivered via osmotic pump calibrated to release half‑life‑matched doses; timing validated by blood NAD+ profiling.
• Readouts: autophagic flux (LC3‑II/I ratio with bafilomycin A1 chase, p62 levels) harvested at ZT4, ZT12, ZT20; lysosomal function (cathepsin activity, LysoTracker); circadian clock gene expression (qPCR for Bmal1, Per2); frailty index and survival monitored up to 30 months.
• Analysis: Two‑way ANOVA for timing × treatment effects on flux metrics; Kaplan‑Meier survival with log‑rank test for lifespan differences.

If night‑specific SIRT1 elevation yields significantly greater autophagic flux and lifespan extension than equivalent day‑timed or constant dosing, the hypothesis will be supported. Conversely, absence of added benefit would falsify the claim that circadian‑timed SIRT1 amplification provides supra‑additive geroprotection beyond fasting duration alone.