Hypothesis

Individuals with longer baseline REM latency require a higher dose of morning light (≥30 min outdoor light before 10 a.m.) to normalize sleep latency after evening screen exposure, whereas those with short REM latency achieve the same benefit with lower light doses.

Mechanistic Rationale

Evening screen use suppresses melatonin via melanopsin‑containing retinal ganglion cells, shifting the circadian phase later [https://www.health.harvard.edu/healthy-aging-and-longevity/blue-light-has-a-dark-side], [https://pmc.ncbi.nlm.nih.gov/articles/PMC3047226/]. The magnitude of this shift depends on the existing circadian timing, which is reflected in sleep architecture. REM latency—the interval from sleep onset to first REM episode—is inversely related to circadian phase advance capacity: longer REM latency indicates a later circadian phase and a reduced ability to advance the clock in response to light [https://academic.oup.com/sleep/article/46/Supplement_1/A256/7181854]. Morning light advances the sleep midpoint by suppressing melatonin and raising cortisol, an effect that scales with light duration and intensity [https://pmc.ncbi.nlm.nih.gov/articles/PMC12502225/]. Therefore, participants with longer REM latency start from a more delayed phase and need a larger photon input to achieve the same advance as those with shorter REM latency.

Testable Predictions

1. In a within‑subject design, administering a fixed 30‑minute morning light session after a standardized evening screen period will reduce sleep latency more effectively in participants with baseline REM latency <20 min than in those with REM latency >30 min.
2. A dose‑response curve will show that the required morning light duration to return sleep latency to baseline increases linearly with baseline REM latency (e.g., each 5‑min increase in REM latency adds ~5 min of needed morning light).
3. Melatonin suppression measured in saliva will correlate negatively with the achieved phase advance, and this relationship will be moderated by REM latency.

Experimental Design

• Recruit 60 healthy adults stratified by baseline REM latency (low, medium, high) using a single night of polysomnography.
• Each participant completes two counterbalanced conditions: (a) evening screen exposure (2 h of blue‑light enriched tablet use ending 30 min before bedtime) followed by a fixed 30‑minute outdoor light session the next morning; (b) same evening screen exposure followed by a personalized morning light session whose duration is titrated based on baseline REM latency (e.g., 20 min, 30 min, 40 min for low, medium, high groups).
• Primary outcome: sleep latency measured via the Multiple Sleep Latency Test (MSLT) the following day [https://sleepeducation.org/patients/multiple-sleep-latency-test/].
• Secondary outcomes: salivary melatonin amplitude, cortisol awakening response, and self‑reported sleep quality.
• Use mixed‑effects models with REM latency as a continuous predictor, light dose as a fixed effect, and participant as a random effect.

Potential Implications

If confirmed, this hypothesis would enable precision circadian interventions: a quick assessment of REM latency from a routine PSG or a validated machine‑learning model [https://academic.oup.com/sleep/article/46/Supplement_1/A256/7181854] could prescribe the exact morning light dose needed to offset individual screen‑induced sleep delays, moving beyond one‑size‑fits‑all recommendations.