When you are outside on a bright day with no eyewear, it can be difficult to keep your eyes open. it makes you feel uncomfortable and sometimes you may even have a headache. That's when you know you're getting too much sunlight, then it's time to go indoors.
Unfortunately, inferior sunglasses brings you comfort, but does nothing good for your eyes. You're getting rid of the warning signal. You stay outside a lot longer than you would have without your sunglasses, increasing your sun exposure.