Daylight exposure improves sleep and subsequent sleepiness

Study Objectives: The causal effects of daylight exposure on sleep remain poorly understood, particularly under real-world conditions. This study aimed to address this gap by examining the effects of an afternoon daylight intervention on subsequent sleep. Methods: Ten healthy participants (4 males; 19 to 28 years) completed two randomized 3-h afternoon outdoor walks under high-intensity light (HIL; clear goggles) or attenuated light (AL; neutral-density-filtered goggles; ~10 photopic lux) conditions. Light exposure was centered around the estimated crossover between the phase-advancing and phase-delaying portions of the light phase-response curve, individually determined from baseline dim light melatonin onset (DLMO). Subsequent nocturnal sleep was assessed using at-home polysomnography (PSG), and subjective sleepiness and sleep quality were assessed using the Karolinska Sleepiness Scale and Groningen Sleep Quality Scale, respectively. Differences between conditions and across time were examined using linear mixed-effects models. Results: There were no significant differences between conditions in DLMO timing or the phase angle between DLMO and sleep onset, indicating that the light manipulation did not produce a detectable shift in circadian phase. Sleep duration and self-reported sleep quality were also unchanged. However, nighttime delta power accumulated more rapidly following HIL exposure, accompanied by greater NREM sleep and reduced wakefulness after sleep onset (all p < 0.01). Participants were also significantly less sleepy upon awakening following HIL compared with AL exposure (p < 0.05). Conclusions: High-intensity natural daylight exposure may promote greater sleep intensity and reduce sleepiness upon awakening. These findings highlight a potential role for daytime light exposure in modulating subsequent sleep physiology and next-morning sleepiness, independent of changes in circadian phase.