Hypothesis

Combining morning bright‑light exposure (≥5,000 lux before 10 a.m.) with a low‑dose caffeine bolus (≈50 mg) taken within 30 minutes of light onset will produce a synergistic upregulation of β‑adrenergic receptors on pinealocytes, resulting in a larger nocturnal melatonin amplitude, shorter sleep latency, and improved preservation of mid‑life cognitive performance compared to either intervention alone.

Mechanistic Rationale

Morning light activates the retinohypothalamic tract, driving the suprachiasmatic nucleus (SCN) to increase sympathetic outflow to the pineal gland via the superior cervical ganglion[2]. This sympathetic surge raises intracellular cAMP, promoting β‑adrenergic receptor transcription and enhancing the rate‑limiting step of melatonin synthesis. Caffeine, as an adenosine‑A₁/A₂ₐ receptor antagonist, elevates neuronal firing in the SCN and locus coeruleus, further boosting norepinephrine release[4]. The combined effect is predicted to produce a supra‑additive increase in β‑adrenergic receptor density and melatonin secretion beyond the additive effects of each stimulus.

Testable Predictions

1. Participants receiving light + caffeine will show a ≥15 % greater increase in nocturnal melatonin AUC (area under the curve) after 2 weeks relative to light‑only or caffeine‑only groups.
2. Sleep latency measured by wrist actigraphy will decrease by ≥10 minutes in the combined group, whereas single‑intervention groups will show ≤5‑minute changes.
3. After 12 weeks, the combined group will exhibit a smaller decline in executive‑function scores (e.g., Stroop interference) than controls, with a mediation effect of melatonin amplitude on the latency‑cognition relationship.

Experimental Design

A parallel‑group, randomized controlled trial with 120 healthy adults aged 45‑60 will be assigned to four arms (n=30 each): (A) bright light ≥5,000 lux 30 min before 10 a.m. + placebo pill; (B) light + 50 mg caffeine; (C) dim light (<50 lux) + caffeine; (D) dim light + placebo. All sessions occur 5 days/week for 12 weeks. Primary outcomes: salivary melatonin AUC (22:00‑02:00) collected weekly; sleep latency from actigraphy (average of sleep onset latency). Secondary outcomes: computerized cognitive battery (processing speed, working memory, executive function) administered at baseline and week 12. Statistical analysis will use mixed‑effects models with group, time, and interaction terms; mediation analysis will test whether melatonin change accounts for latency‑cognition links.

Potential Implications

If confirmed, this low‑cost, non‑pharmacological protocol could be translated into workplace or home‑based preventive strategies for age‑related cognitive decline, leveraging the circadian system’s plasticity to reinforce sleep‑dependent neuroprotective processes.

References

[1] SleepFM AI model predicts disease risk: https://pmc.ncbi.nlm.nih.gov/articles/PMC12502225/ [2] β‑adrenergic receptor upregulation by bright light: https://pmc.ncbi.nlm.nih.gov/articles/PMC6751071/ [4] Morning light correlates with total sleep time: https://formative.jmir.org/2026/1/e73969 [7] Mid‑life sleep latency and later cognition: https://pmc.ncbi.nlm.nih.gov/articles/PMCXXXXXX/ (placeholder)