Hypothesis

Timed morning light exposure preserves enteric nervous system (ENS) neurons via central circadian output, thereby preventing microbiome‑driven inflammaging and extending lifespan.

Mechanistic Rationale

Morning light activates ipRGCs, shifting the SCN clock and boosting sympathetic and parasympathetic outflow to the gut (Morning light entrains central circadian clocks). This autonomic tone won't just increase motility; it releases norepinephrine and acetylcholine that stimulate enteric glia to secrete BDNF and GDNF, factors known to support ENS neuron survival. When light input is mistimed or absent, SCN output drops, reducing trophic support and accelerating the 30‑60% ENS neuron loss seen in aged mice (Aging enteric nervous system shows 30‑60% neuronal loss). A depleted ENS slows motility, alters mucus secretion, and shifts the microbiota toward taxa that produce senescence‑inducing metabolites like phenylacetic acid (Age‑associated gut microbiota shifts elevate phenylacetic acid). The resulting endothelial senescence and systemic inflammation feed back to the brain, creating a vicious loop that shortens healthspan. In contrast, robust morning light sustains ENS integrity, keeps the microbiome in a youthful configuration, and blocks the gut‑to‑brain senescence cascade.

Testable Predictions

1. Animal test – Old mice receiving 30 min of 10 000 lux light at ZT0 will show (a) higher ENS neuron counts (via HuC/D staining), (b) increased colonic BDNF/GDNF levels, and (c) a microbiome profile enriched in short‑chain fatty acid producers compared with age‑matched controls kept in dim light.
2. Human pilot – Older adults prescribed 30 min of outdoor morning light for 8 weeks will exhibit (a) faster gut transit (measured by wireless motility capsule), (b) elevated plasma BDNF, and (c) reduced circulating phenylacetic acid relative to a wait‑list group.
3. Intervention specificity – Blocking β‑adrenergic signaling in the gut with propranolol will abolish the light‑induced ENS protection, confirming autonomic mediation.
4. Rescue experiment – Chemogenetic activation of ENS neurons in light‑deprived aged mice will normalize motility and lower phenylacetic acid, proving that ENS preservation is sufficient to mitigate microbiome‑derived senescence.

Potential Challenges

• Light exposure may affect behavior (activity, feeding) independently of circadian pathways; controlling for these confounds requires pair‑feeding and activity‑matching designs.
• ENS neuron quantification is labor‑intensive; using transgenic reporters (e.g., Sox10‑GFP) can streamline counts.
• Translating mouse light intensity to humans needs careful calibration to avoid retinal damage; using certified light boxes with UV filters mitigates risk.