Morning Light Enhances Glymphatic Clearance to Boost Sleep Efficiency

Morning light exposure within the first hour of waking activates melanopsin‑expressing retinal ganglion cells, phase‑advancing the suprachiasmatic nucleus and suppressing melatonin [[https://pmc.ncbi.nlm.nih.gov/articles/PMC12989132/]]. Beyond circadian alignment, this acute photic signal triggers a sympathetic surge that elevates norepinephrine release in the perivascular space of the brain. Norepinephrine drives aquaporin‑4 (AQP4) polarization toward astrocytic endfeet, a key step that increases glymphatic convective flow during subsequent sleep [[https://www.nature.com/articles/nn.4572/]]. Enhanced glymphatic clearance removes metabolic waste such as amyloid‑β and lactate, reducing interstitial pressure and promoting smoother transitions between NREM stages. We hypothesize that individuals who receive ≥3000 lux melanopic equivalent daylight illuminance (mEDI) for 60 minutes immediately after awakening will show a ≥15 % increase in overnight glymphatic influx (measured by CSF‑to‑serum ratio of inert tracer) and consequently achieve sleep efficiency >90 % (sleep time/time in bed) compared with a dim‑light control (<50 lux).

Testable prediction: In a within‑subject crossover design, participants undergo two 3‑day conditions separated by a washout. Condition A: bright outdoor light (>3000 lux) or blue‑enriched indoor light matched for mEDI within 5 minutes of waking. Condition B: identical timing but with light filtered to <50 lux. On night 3 of each condition, intrathecal infusion of a fluorescent tracer quantifies glymphatic influx via MRI‑based kinetic modeling, while polysomnography derives sleep efficiency, wake after sleep onset, and stage percentages. We expect a significant positive correlation between mEDI‑driven glymphatic influx and sleep efficiency (r > 0.4, p < 0.05) and a negative correlation with wake after sleep onset (r < ‑0.3, p < 0.05).

Falsifiability: If bright morning light fails to elevate glymphatic influx or does not improve sleep efficiency relative to dim light, the hypothesis is refuted. Conversely, a null effect on glymphatic metrics despite improved efficiency would suggest alternative mechanisms, prompting investigation of non‑glymphatic pathways (e.g., metabolic rate changes). This mechanism links photic entrainment to homeostatic clearance, offering a concrete target for optimizing recovery through timed light exposure.