Virtually all parts of the eye, including the skin around it can be damaged by excessive sun exposure, but your eyes also need exposure to full spectrum light to function optimally.
There is no need for you to fear the sun, and as matter of fact, a lot of health issues are caused because of an extreme lack sunlight. Though it is proven that lack of sunlight leads to OSTEOPOROSIS and DEPRESSION.
Here are the reasons:

• We need sunlight to put our hormones into place: We don't need to block the natural light from the eye with our source
  sunglasses, though we need to avoid the midday sun, We humans we thrive during the day light. even though the sunlight that reaches your eyes stimulates the hypothalamus in the brain which is connected to the pineal gland, and the pineal gland is responsible for regulating the sleep and wake-up cycle. And if you always put on an unnatural tint the rate of the sunlight will not be that much and it can affect according to the above explanation. source
• In bright sunlight the eyes will naturally try to protect themselves from too much ultraviolet rays getting into the eye by shrinking the size of the pupils. But when sunglass is been put on then it creates darkness over your eye, in which the pupil is widely open which allow even more potential harmful (UV)light which is dangerous. In short wearing of sunglasses with improper ultraviolet can be far worse than not putting it on at sll. To those that are wearing it for the sake of fashion should try and avoid it. Source
• Less adaptable to light and dark: If the eye are not in use then you lose them, because the eye operate like the other organs in the body. If the eyes is not been exposed to the light consistently then your ability to adapt to the light and infact vision will surely decline.
• As we know that the eyes is designed to interact with the sun light, therefore putting on/wearing the sun glasses for a long period of time is like putting the eye through stress, since the eye is force to operate through un natural 'tint' from sunglasses. Through this, your eyes are been forced to work harder than normal when using the sunglasses which will eventually lead to eye fatigued.(i.e the eye will be weak and lose strength. source
  Actually am not saying that putting on sunglass is bad but at the same time is also bad. What am trying to picture out is that you need to be aware of the stress you put your eye through when putting on sunglasses all in the name of fashion. And if you want to put it on just remember to put it on balance so that it won't affect you much
  Stop by and READ for it will be of advantage to every one.
  Reference