Photochromic fitover sunglasses react automatically to the available sunlight. On overcast days the lens tints are medium to light. On sunny days they darken to a comfortable sunglass tint and when you go inside, they lighten up. Since they darken and lighten in response to the amount of sunlight you can wear them from dawn to dusk and never have to take them off. They're also known as photochromatic, transitional, day to night or reactolite fit over sunglasses.
Please note, photochromic lenses react to UV rays and do not work when worn behind a helmet visor or motor vehicle windscreen that filters out UV light.