Hypothesis

Exposure to bright morning sunlight within the first hour after waking will attenuate the performance and sleep‑quality deficits associated with sleeping on a fixed 8‑hour schedule that opposes an individual's chronotype, making chronotype‑aligned sleep unnecessary for optimal outcomes when morning light is sufficient.

Mechanistic Rationale

Morning sunlight activates melanopsin‑containing ipRGCs, driving a robust phase‑advancing signal to the suprachiasmatic nucleus (SCN) and increasing the amplitude of the circadian rhythm [2,3]. Higher amplitude enhances the SCN’s ability to entrain to external cues, thereby reducing the impact of internal‑external phase misalignment [1]. In contrast, evening blue‑light blockade only preserves melatonin secretion without boosting amplitude, explaining its modest effects [4,5]. Thus, a strong morning photic signal can compensate for a misaligned sleep window by stabilizing the downstream circadian output that governs melatonin onset, cortisol awakening response, and sleep propensity.

Predictions

1. Participants receiving morning sunlight will show similar sleep onset latency, total sleep time, and next‑day psychomotor vigilance test (PVT) scores regardless of whether they sleep according to their chronotype or a fixed early schedule.
2. The advantage of chronotype‑aligned sleep over fixed scheduling will diminish as morning light dose increases, becoming non‑significant at ≥10 min of direct sunlight ≥300 lux.
3. Adding blue‑blocking glasses in the evening will not provide additional benefit beyond morning sunlight and dim evening light.

Experimental Design

A 2 × 2 factorial RCT (n = 120) stratified by baseline chronotype (early vs. late) will assign participants to:

• Morning sunlight (10 min outdoor light ≥300 lux within 60 min of waking) vs. sham (indoor low‑light <50 lux).
• Sleep schedule: chronotype‑aligned (self‑selected bedtime yielding ≥7 h) vs. fixed early (lights‑out at 23:00, wake at 07:00). All participants will avoid screens after 21:00 and use identical dim‑light (<30 lux) conditions; blue‑blockers will be provided to a subset to test additivity. Primary outcomes: melatonin onset (DLMO), sleep onset latency, total sleep time (actigraphy), and PVT reaction time after 3 nights of adherence. Secondary: mood (PANAS), metabolic markers (fasting glucose).

Potential Outcomes and Falsifiability

If morning sunlight does not reduce the performance gap between chronotype‑aligned and fixed schedules (i.e., the interaction term is non‑significant and the fixed‑schedule group under sunlight still shows worse DLMO, longer SOL, and slower PVT than the chronotype‑aligned group), the hypothesis is falsified. Conversely, a non‑significant difference between schedule groups under sunlight, coupled with a significant main effect of morning light on amplitude markers (e.g., higher cortisol awakening response), would support the hypothesis.