Doctors do not recommend sunglases when the weather is cloudy but at the same time they say that UV rays from the sun can damage eysight even when weather is cloudy.

What is going on?

You may be experiencing the "money Vs medicine" syndrome. The doctors tell you not to wear sunglasses during cloudy days (and at night) because if they did and you had an accident you would sue them. As far as I know the UV does not change due to light conditions.

The other possibility is, your body needs a certain amount of exposure to light to keep the old body clock set right. So you may not get the full light dose on a cloudy day if you wear sunglasses. (I head something like 4 hours of very bright light are required to keep the clock ticking, on a daily basis.)

You need to wear sunglasses during cloudy days, for the UV rays are bad for your eyes. You may opt to use a brown or yellow tinted lens to allow better vision during low light. Grey and green lenses are not recommended for low light. Also, UV is increased during the colder seasons, so continue wearing them all year round.

Selecting a good pair of sunglasses:
*avoid cheap sunglasses
*make sure they state 100%UV protection (if only "UV Coated" is on the sticker, they may contain 80% or less)
*sensitive eyes - use grey or green or dark brown
*polarized lenses are best for driving and being around water
*before purchasing, make sure you try them outside, allowing you the chance to see if you really like the tint
*avoid very dark lenses for safety reasons
*make sure the frame is good quality
*backside Anti-reflective coating is a plus
*Great Brands - Maui Jim, Revo, Serengetti, RayBan (some are expensive, but are definitely worth every penny)