Sunlight triggers a cutoff of melatonin, bringing on wakefulness. (Indoor light--particularly the light from tablets and smartphones--can also suppress melatonin, but nowhere near as dramatically as sunlight.) This is why night shift workers who drive home in the morning through sunlight and then struggle to fall asleep may find relief by buying amber-lensed Bono-style glasses that block the sun's blue light wavelengths. NSMRL