Retinol is a form of Vitamin A, and even in the lighter formulations available over the counter it makes your skin highly sensitive to UV rays. Retinol causes the surface skin cells to turn over more quickly (thus improving the appearance of fine lines), which means the newly-exposed skin is vulnerable to UV damage. So if you've been using retinol nightly but not practicing adequate UV protection every day, it's entirely possible your skin has "tanned," or been darkened by unprotected exposure to sunlight. This is especially true for people with very light skin. It's not the nightly use of the retinol product itself that is responsible for the darkening, but the fact that you probably haven't been scrupulous about applying a broad-spectrum sunscreen (one that protects from UVA and UVB rays) every morning, no matter what the weather. Choose a sunscreen without added retinol, and apply it thoroughly to exposed skin. Reapply during the day, especially if you work out or swim. While using retinol (or stronger prescription-based formulations such as retinoids and Retin-A), you should also take care to avoid prolonged exposure to sunlight or even strong interior lighting. Protective hats and sunglasses are important. In the meantime, you might ask your dermatologist about a gentle skin brightener to apply nightly until the tanning has cleared up.
