Synergistic morning light and evening warming enhances circadian amplitude and slow-wave sleep

Morning bright light (≥10,000 lux) activates ipRGCs that signal the SCN to suppress melatonin and advance circadian phase, while evening distal skin warming (31.7‑34.6°C) facilitates the ~1 °C core body temperature drop needed for sleep onset and increases slow‑wave sleep probability. We hypothesize that combining these two stimuli produces a synergistic amplification of circadian amplitude beyond the sum of their individual effects. Specifically, morning light‑driven SCN output increases sympathetic tone to cutaneous vasculature, priming the skin for enhanced heat loss; evening warming then capitalizes on this primed state to provoke a larger and faster core temperature decline, which in turn reinforces SCN signaling through thermosensitive feedback loops. This bidirectional coupling should raise melatonin suppression magnitude, deepen the nocturnal temperature trough, and increase the proportion of slow‑wave sleep in healthy adults.

To test this, we propose a randomized, crossover study with four 3‑day conditions separated by ≥1‑week washout: (1) morning bright light (10,000 lux, 30 min within 30 min of waking), (2) evening distal warming (water‑perfused suit at 33 °C from 20:00 to 22:00), (3) combined morning light + evening warming, and (4) dim light (<50 lux) + neutral temperature suit (control). Participants (n=30, ages 20‑35) will wear actigraphs and continuous core temperature pills, provide salivary melatonin samples every 30 min from 18:00 to 02:00, and undergo polysomnography on night 3 of each condition. Primary outcomes are melatonin AUC suppression, nadir core body temperature, and slow‑wave sleep percentage. We predict that the combined condition will produce melatonin AUC suppression >20 % greater than either single stimulus (p<0.01), a core temperature nadir ≥0.3 °C lower than control (p<0.01), and slow‑wave sleep proportion increased by ≥15 % relative to control (p<0.01). Failure to observe these differences would falsify the hypothesis of synergistic interaction.