Photochromic lenses darken on exposure to sunlight, which makes them perfectly suitable for everyday use both outdoors and indoors. They reduce glare and protect your eyes from harmful UV rays not only on bright sunny days but also when it is cloudy.
However, as said above photochromic lenses react to a direct contact with UV rays, which means there will be cases when you will need a pair of prescription sunglasses. For instance, a car windshield blocks UV rays away, hence photochromic lenses will not darken while it might be very bright and sunny. Also when it’s extremely hot and sunny, photochromic eyeglasses may not tint as much as sunglasses.