Evening blue light suppresses melatonin and delays circadian phase, but the ipRGC pathway also conveys light information to peripheral organs via distinct neural routes. We hypothesize that delivering short, high-intensity blue-light pulses (≈480 nm, 200 lux, 2 min) at specific phases of the peripheral clock—identified by the rising edge of PER2 expression in liver and adipose tissue—will entrain those clocks while preserving overall melatonin secretion. This hypothesis extends the current view that light exposure must be either avoided entirely or applied continuously in the morning, by proposing a temporally precise “micro‑entrainment” strategy that leverages ipRGCs’ capacity to drive heterogeneous downstream signaling.

Mechanistic rationale: ipRGCs project to the SCN via glutamatergic synapses and to the paraventricular nucleus (PVN) and intergeniculate leaflet (IGL) via peptidergic signals (e.g., VIP, NPY). The PVN‑IGL route preferentially modulates autonomic output to liver and adipose tissue, influencing clock gene expression without strongly affecting the melatonergic pathway. Brief, intense pulses are sufficient to trigger calcium spikes in ipRGCs that favor peptidergic release over sustained glutamatergic firing, thereby shifting the balance toward peripheral entrainment. Supporting evidence shows that melanopsin‑driven IPSCs can be decoupled from SCN firing patterns by altering stimulus duration and intensity (see PMID:27992553).

Testable predictions:

1. In a within‑subject crossover trial, participants receiving three 2‑minute blue-light pulses at 02:00, 04:00, and 06:00 h (based on individual DLMO) will show a ≥30 % increase in hepatic PER2 amplitude measured via fasting blood transcriptomics, compared to a control condition with dim red light.
2. Melatonin AUC across the night will not differ significantly between pulse and control conditions (p > 0.05), indicating preserved endocrine signaling.
3. Post‑prandial glucose tolerance will improve by ~15 % after one week of pulse exposure, reflecting enhanced peripheral clock‑metabolism coupling.

Falsifiable outcome: If peripheral PER2 rhythms fail to amplify or melatonin suppression occurs despite the pulse protocol, the hypothesis that ipRGC peptidergic signaling can be selectively engaged would be refuted, suggesting that any ipRGC activation inevitably drives global circadian shifts.

Experimental design: Recruit 20 healthy adults with habitual mid‑sleep >02:00 h. Determine DLMO via salivary melatonin under dim‑light (<5 lux). Randomize to pulse or sham (identical timing, <1 lux red light) for 7 days, then crossover after a 2‑week washout. Collect fasting blood at 08:00 h for RNA‑seq of clock genes, hourly melatonin profiles from 20:00–08:00 h, and an oral glucose tolerance test on day 8. Use mixed‑effects models to test interaction effects.

Broader impact: Confirming that discrete light pulses can fine‑tune peripheral clocks would enable personalized lighting interventions for shift workers or individuals with metabolic dysfunction, minimizing side effects on sleep‑related hormones while improving glucose homeostasis.