Daylight exposure improves sleep and subsequent sleepiness

High-intensity natural daylight during a randomized 3-h afternoon outdoor walk increased nighttime delta power accumulation and reduced wakefulness after sleep onset in 10 healthy adults.

• Why it matters: The causal effects of real-world daylight on sleep physiology and next-morning sleepiness are poorly understood, especially when circadian phase shifts are not evident.
• What they did: Ten healthy participants (19 to 28 years) completed two randomized afternoon outdoor walks under high-intensity light (~clear goggles) versus attenuated light (~10 photopic lux), with at-home PSG and Karolinska Sleepiness Scale assessments analyzed using linear mixed-effects models.
• The result: High-intensity light showed faster delta power accumulation (all p < 0.01), greater NREM sleep and reduced wakefulness after sleep onset, and participants were less sleepy upon awakening versus attenuated light (p < 0.05).