Morning bright light (≥2,500 lux within 30–60 min of waking) entrains the suprachiasmatic nucleus (SCN) and advances dim‑light melatonin onset (DLMO) by 10–45 min, while evening bedroom cooling to 18–19 °C enhances slow‑wave sleep and REM proportion. Both interventions act on distinct pathways—light via melanopsin‑SCN signaling and temperature via peripheral clock modulation and hypothalamic thermosensitive neurons—but their combined impact on circadian amplitude has not been quantified. We hypothesize that simultaneous application of morning bright light and evening cooling produces a supra‑additive increase in circadian rhythm amplitude, reflected by larger melatonin amplitude, steeper cortisol awakening response, and greater heart‑rate variability (HRV) recovery than the sum of each intervention alone.

Mechanistically, morning light strengthens SCN output to the dorsomedial hypothalamus (DMH) and paraventricular nucleus (PVN), boosting sympathetic tone during the day. Evening cooling activates preoptic area warm‑sensitive neurons that inhibit the DMH‑PVN axis, promoting parasympathetic rebound at sleep onset. When the SCN is already primed by strong morning light, the evening cooling‑induced parasympathetic surge may more effectively entrain downstream autonomic and hormonal rhythms, amplifying the overall circadian signal. This cross‑talk could also enhance clock‑gene expression (e.g., PER2, BMAL1) in both central and peripheral tissues beyond what either stimulus achieves independently.

To test this, we propose a randomized, crossover trial with four ‑day conditions in healthy adults: (A) morning bright light (10,000 lux, 15 min) only, (B) evening cooling (bedroom 18 °C, 8 h) only, (C) combined morning light + evening cooling, and (D) dim light (<50 lux) + neutral temperature (22 °C) control. Each condition lasts 3 days with a 2‑day washout. Primary outcomes are melatonin amplitude (area under the curve), cortisol awakening response slope, and nightly HRV (RMSSD). Secondary outcomes include DLMO shift, sleep architecture (REM, SWS %), and self‑reported alertness.

Prediction: Condition C will show a melatonin amplitude increase ≥30 % greater than the additive sum of conditions A and B, a cortisol slope steepening ≥20 % beyond additivity, and HRV recovery ≥25 % above the summed individual effects. If the combined condition yields only additive or sub‑additive changes, the hypothesis is falsified. This design directly tests whether timed light and thermal cues interact synergistically to strengthen circadian amplitude, offering a mechanistic bridge between environmental entrainment and autonomic regulation.