Hypothesis: Exposure to morning light at intensities above 4,000 lux within the first 30 minutes of waking increases next-night slow-wave sleep (SWS) proportion by accelerating adenosine clearance and enhancing thalamocortical GABAergic tone.

Rationale: Morning light activates melanopsin retinal ganglion cells, signaling the suprachiasmatic nucleus to suppress melatonin and raise cortisol. This photic arousal elevates neuronal metabolic demand during wakefulness, increasing adenosine production. Simultaneously, bright light stimulates astrocytic glycogenolysis and promotes adenosine uptake via equilibrative nucleoside transporters, lowering extracellular adenosine by evening. Reduced evening adenosine diminishes homeostatic sleep pressure, setting the stage for a rebound increase in SWS when adenosine reaccumulates during the night. Additionally, SCN‑driven activation of the ventrolateral preoptic area boosts GABA release onto thalamic relay neurons, stabilizing sleep spindles and facilitating SWS onset.

Predictions: 1) Participants receiving ≥4,000 lux outdoor light for 10 min within 30 min of wake will show a significant increase in SWS percentage (measured by polysomnography or a validated wearable) the following night compared with <1,000 lux control. 2) The magnitude of SWS increase will correlate with reduction in evening salivary adenosine levels. 3) Pharmacological blockade of GABA_A receptors in the thalamus (using a low dose of bicuculline administered intranasally in a crossover design) will abolish the light‑induced SWS enhancement, confirming thalamic GABAergic mediation.

Falsifiability: If high‑intensity morning light fails to raise SWS or does not alter evening adenosine, or if thalamic GABA blockade does not attenuate the effect, the hypothesis is refuted.

Methods: Within‑subject crossover, n=30 healthy adults, random assignment to high‑lux (≥4,000 lux) or low‑lux (<1,000 lux) morning light sessions for three consecutive days, with a washout week. Light intensity logged via calibrated wearable photometers. Saliva collected at wake, noon, and evening for adenosine assay (enzymatic). Sleep recorded via polysomnography or validated EEG wearable (e.g., Eight Sleep algorithm) to derive SWS proportion. Statistical analysis using mixed‑effects models with light condition, adenosine change, and GABA blockade as fixed effects, subject as random effect.

Link to cited works: The necessity of outdoor light for circadian entrainment is established [1]; phase‑advancing effects of >3,000 lux light are documented [2]; deep‑learning sleep staging improves SWS detection [3]; temperature and consistency recommendations support sleep optimization [4]; energy‑minimization models show how subcortical circuits can be tuned for better stage sequencing [5].

Novel mechanism: Connects photic arousal to metabolic clearance of adenosine and to GABAergic gating of thalamic rhythms, providing a testable bridge between morning light exposure and next‑night SWS improvement.