Bright light contracts your pupils. The smaller diameter leads to a greater effective depth of focus. For people who have difficulty focusing on close items,

Bright light contracts your pupils. The smaller diameter leads to a greater effective depth of focus. For people who have difficulty focusing on close items, this can help, I believe. It doesn't mean dim light is "destroying" your eyesight. However, it could be reducing the effective depth of focus, making it difficult to see what you're looking at clearly.