Hypothesis

Receiving 30 minutes of ~5000 lux broad‑spectrum light within 5 minutes of waking reduces the magnitude of melatonin suppression caused by subsequent evening exposure to 550 lux blue‑enriched light (~460‑480 nm) by at least 30 % compared with evening exposure without prior morning light.

Mechanistic Rationale

Morning light drives a rapid dephosphorylation of melanopsin in ipRGCs, shifting the photopigment to a low‑sensitivity state that persists for several hours【1】. This adaptive change decreases the gain of the ipRGC‑to‑SCN pathway, meaning that photons arriving later in the day produce a smaller intracellular calcium signal and consequently less inhibition of pineal melatonin synthesis. Evening blue light normally exploits the high‑sensitivity, phosphorylated melanopsin state to delay the circadian clock【2】. If the morning‑induced dephosphorylated state is still present, the same evening photon flux will elicit a weaker response, thereby attenuating melatonin suppression and reducing sleep onset latency.

Predictions

1. Salivary melatonin concentrations measured 30 minutes after the evening light block will be significantly higher in the morning‑light condition than in the control condition.
2. Subjective sleep latency (via sleep diary or actigraphy) will be shortened by ≥2 minutes in the morning‑light condition.
3. The effect will diminish when the interval between morning and evening light exceeds 4 hours, reflecting the time course of melanopsin re‑phosphorylation.

Experimental Design

A within‑subject, crossover study with 30 healthy adults (age 18‑35) who are not shift workers. Each participant completes two 3‑day sessions separated by a 1‑week washout. In the experimental session, subjects receive 30 minutes of 5000 lux white LED light (broad spectrum, 400‑700 nm) immediately upon waking (within 5 minutes). In the control session, they remain in dim (<50 lux) indoor lighting for the same period. On each evening, after a fixed wake‑time, participants are exposed to 550 lux blue‑enriched LED light (peak 470 nm) for 6.5 hours starting 2 hours before habitual bedtime. Saliva samples are collected hourly from 2 hours before bedtime until 1 hour after lights‑out to assay melatonin. Sleep latency is assessed via polysomnography on the final night and via wrist actigraphy for the preceding two nights.

Statistical analysis will use paired t‑tests for melatonin area‑under‑the‑curve and sleep latency, with Bonferroni correction for multiple comparisons. A mixed‑effects model will test the interaction between session (morning light vs. control) and time interval (≤4 h vs. >4 h) to verify the predicted decay of the effect.

If the hypothesis is supported, it would demonstrate that timing‑dependent ipRGC plasticity can be harnessed to mitigate the disruptive effects of evening artificial light, offering a low‑cost, behaviorally grounded strategy for improving sleep onset in populations exposed to nighttime screen use.