If you're slathering on retinol at all hours of the day in hopes of clear, youthful skin, dermatologist Mona Gohara, MD, advises you stop. "Retinol degrades in sunlight," Gohara said. "Ultraviolet light destabilises it so it isn't as effective." Because of the negative effect sunlight has on retinol, she recommends that you only use the powerhouse ingredient at night. Not only will it work its magic while you sleep, but it'll also be most effective and safe after it's been stored in a dark, cool environment.