Hypothesis

Aligning bright‑light exposure to an individual’s wake time (chronotype‑personalized morning light) and timing evening blue‑light blocking to the individual’s endogenous dim‑light melatonin onset (DLMO) window produces greater advances in circadian phase, improvements in insulin sensitivity, and enhancements in cognitive performance than fixed‑clock‑time interventions.

Mechanistic Rationale

Morning light activates melanopsin‑containing intrinsically photosensitive retinal ganglion cells (ipRGCs), which signal the suprachiasmatic nucleus (SCN) to suppress melatonin and elevate cortisol, thereby shifting the circadian clock earlier [1][2]. The SCN’s sensitivity to light peaks within the first 30‑60 minutes after awakening [2]; delivering light outside this window reduces the efficacy of phase‑shifting signals. Consequently, delivering a fixed 06:00‑06:30 h light pulse to a late chronotype whose biological morning occurs later misses the optimal ipRGC activation period, yielding a smaller phase advance.

Evening blue‑light blocking works by preventing ipRGC stimulation that would otherwise delay melatonin secretion [3]. The timing of melatonin onset varies substantially across individuals (DLMO ranging roughly from 20:00 to 02:00). Applying a uniform blocking interval (e.g., 20:00‑22:00) may either start too early (producing unnecessary behavioral inconvenience) or too late (failing to attenuate the light‑driven delay). Aligning the block to the individual’s DLMO‑2h to DLMO+2h window maximally reduces melanopsin‑driven phase‑delaying input while preserving peripheral light cues necessary for retinal health.

Beyond phase shifting, circadian alignment influences hepatic glucocorticoid receptor signaling and PER2‑mediated regulation of gluconeogenesis, offering a mechanistic link to improved insulin sensitivity [4]. Cognitive benefits arise from stabilized alertness‑promoting orexin signaling and reduced sleep inertia when the internal clock matches external demands.

Testable Predictions

1. Participants receiving chronotype‑personalized morning light will exhibit a larger advance in DLMO (≥ 15 min) compared with those receiving fixed‑time morning light.
2. Evening blue‑light blocking timed to individual DLMO will increase melatonin amplitude and reduce sleep latency more effectively than uniform blocking.
3. The combined personalized light protocol will produce greater improvements in HOMA‑IR (≥ 10 % reduction) and faster reaction times on the Psychomotor Vigilance Task (≥ 15 ms improvement) than fixed interventions.
4. If personalized interventions confer no significant advantage over fixed‑clock approaches, the hypothesis is falsified.

Experimental Design

A within‑subject, crossover trial with 60 adults stratified by chronotype (early, intermediate, late) based on actigraphy‑derived mid‑sleep. Each participant completes four 2‑week conditions in random order, separated by 1‑week washout:

• Fixed morning light: 5000 lux, 06:00‑06:30 h.
• Personalized morning light: 5000 lux, starting 30 min after individual wake time (verified by actigraphy).
• Fixed evening block: blue‑light blocking glasses, 20:00‑22:00 h.
• Personalized evening block: glasses from DLMO‑2h to DLMO+2h (determined via salivary melatonin assays). Primary outcomes: DLMO shift (salivary melatonin), HOMA‑IR (fasting glucose/insulin), PVT reaction time, and subjective alertness (VAS). Secondary outcomes: actigraphy‑derived sleep efficiency, mood (PANAS). Mixed‑effects models will test condition effects while controlling for baseline chronotype.

This design directly tests whether matching light interventions to internal timing leverages the known ipRGC‑SCN pathway to yield superior circadian, metabolic, and cognitive outcomes, addressing the evidence gaps identified in current literature.