Core Hypothesis

Sleep disturbance reduces the amplitude of systemic NAD+ rhythms, uncoupling two NAD+-dependent repair processes: astrocytic glymphatic clearance in the brain and autophagy in Zone 3 hepatocytes. This concurrent failure creates a bidirectional vulnerability where neurodegeneration and non‑alcoholic fatty liver disease (NAFLD) co‑progress, rather than arising independently.

Mechanistic Rationale

1. NAD+ as a circadian messenger – Hepatic autophagy genes (Atg5, Ulk1, Foxo3) show ZT6 peaks that depend on NAD+/SIRT1 signaling [2]. Likewise, astrocytic AQP4 polarization and glymphatic influx peak during sleep and are enhanced by SIRT1‑mediated deacetylation of aquaporin channels (studies show NAD+ boosters increase CSF tracer clearance). Sleep loss blunts NAD+ oscillations, lowering SIRT1 activity in both compartments.
2. Lactate‑mediated coupling – During sleep, astrocytes release lactate that travels via the portal circulation to the liver. Lactate activates hepatic SIRT3, which deacetylates and activates LC3‑II, promoting autophagosome formation. When sleep is fragmented, astrocytic lactate efflux falls, reducing hepatic SIRT3 activity and attenuating Zone 3 autophagy specifically.
3. Senocyte accumulation as a read‑out – Failed autophagy lets damaged mitochondria and lipid aggregates persist, driving senescence preferentially in Zone 3 [4]. Senescent hepatocytes secrete SASP factors (IL‑6, TNF‑α) that cross the blood‑brain barrier, suppressing astrocytic NAD+ synthesis via CD38 upregulation, creating a vicious loop.
4. Therapeutic prediction – Reinforcing the NAD+ rhythm (e.g., timed nicotinamide riboside dosing at ZT4‑6) should restore both glymphatic flux (measured by intrathecal CSF‑to‑blood tracer clearance) and hepatic autophagy (measured by LC3‑II/I ratio and mitophagy flux in Zone 3) in sleep‑restricted models.

Testable Predictions

• Prediction 1: In mice subjected to chronic sleep restriction, hepatic NAD+ levels will show a 40% reduction in ZT6 peak compared to controls, concomitant with a 35% drop in astrocytic AQP4 polarization (p<0.01). Restoring NAD+ via timed NR will rescue both markers to ≥90% of baseline.
• Prediction 2: Lactate infusion into the portal vein of sleep‑restricted mice will increase hepatic SIRT3 activity and autophagic flux in Zone 3 by ~50% without altering central NAD+, indicating a liver‑specific arm of the coupling.
• Prediction 3: Genetic ablation of astrocytic LDH5 (limiting lactate release) will reproduce the Zone 3 autophagy defect seen with sleep loss, even when systemic NAD+ is normal.
• Prediction 4: Administering a senolytic (dasatinib+quercetin) at ZT6 will reduce hepatic senescent cell burden and improve glymphatic clearance, supporting the SASP‑mediated feedback.

Falsifiability

If timed NAD+ supplementation fails to improve either glymphatic tracer clearance or hepatic autophagy in sleep‑disrupted animals, or if lactate infusion does not rescue Zone 3 autophagy despite normal NAD+, the hypothesis would be refuted. Likewise, demonstrating that astrocytic lactate manipulation does not affect hepatic SIRT3 activity would invalidate the proposed coupling mechanism.

Broader Implications

This hypothesis reframes sleep not merely as a passive rest state but as a systemic metabolic checkpoint that synchronizes brain‑wide waste removal with liver‑specific autophagy through NAD+ and lactate signaling. It suggests that circadian‑targeted NAD+ boosters or timed lactate mimetics could simultaneously mitigate neurodegeneration and NAFLD, offering a unified approach to comorbid age‑related diseases.