Why does morning sunlight improve sleep?

Morning sunlight improves sleep by setting the circadian clock: it promotes alertness during the day and helps align sleepiness with nighttime. 1

• Morning cortisol: Bright morning light can increase the morning cortisol peak by up to 50%. That supports daytime alertness and mood, while helping cortisol fall later so sleep comes more easily at night. 2 3
• Melatonin: Morning light suppresses lingering melatonin after waking; properly timed light also helps keep the cortisol and melatonin rhythms aligned, with melatonin rising at night. 4 5
• Yellow-blue contrast: At low solar angle, sunlight’s yellow-blue contrast activates eye cells that signal the central clock, helping regulate sleep-wake timing and other body rhythms. 6
• Light-therapy lamps: When sunlight isn’t available, a bright light-therapy lamp can be an alternative, though sunlight is more effective. 7