“Light is a stimulus that can have impacts on health, well-being and performance,” for good and ill, says George Brainard, professor of neurology at Thomas Jefferson University.
Light has this power because it drives our central circadian clock. Ideally, that internal clock produces daytime alertness and nighttime sleepiness on a predictable 24-hour schedule. Receptors in our eyes play key roles, taking their cues from the intensity and wavelengths of light we are exposed to each day and night.
Bright natural daylight is rich in blue wavelengths that can alert us and suppress the release of melatonin, a hormone that helps us fall asleep. When we get bright light, especially bright blue light, at night, it can disrupt sleep and might even contribute to weight gain, cancer susceptibility and other health problems, some studies suggest.