Sunglasses prevent your eyes from sensing sunlight. The more your eyes sense sunlight, the more your body/skin metabolizes Vit D from sunlights. Vit D is very important for the proper functioning of your body.
There's a good basis to your anecdotal observation.
Seafood is a good substitute for non-supplement source of Vit D during the winter months. Apparently the Inuit have kept their dark skin, evolutionarily speaking, because they so much fish.