Bright light is what keeps our circadian rhythms in good order.
“One of the most important things you can do for your sleep is to have a routine to keep your circadian rhythm normal,” says Suh.
Lighting signals your body's circadian rhythm and can have a strong effect on your mood.
circadian cir·ca·di·an (sər-kā'dē-ən, -kād'ē-, sûr'kə-dī'ən, -dē'-)
Relating to biological variations or rhythms with a cycle of about 24 hours.