When it comes to applying a vitamin C serum, the answer is both morning and night, after washing and toning. Vitamin C, unlike sunscreen, moisturizers, or oils, cannot be readily wiped or washed away. It must be re-applied. Vitamin C has many benefits for your skin, including reducing the appearance of fine lines and wrinkles, strengthening nails and hair, and fighting acne. It can also help heal minor injuries from punching bags or yoga mats. Just like any other supplement, too much of a good thing can be harmful, so use caution not to overdose on this essential nutrient. Vitamin C is an antioxidant that helps protect cells from damage caused by free radicals. Free radicals are molecules with an odd number of electrons that cause injury to cells as they try to take their place. The human body produces free radicals, but they are also found in food as part of normal metabolism. Antioxidants work with free radicals to reduce their impact on cells. Dietary antioxidants include vitamins A, C, E, and beta-carotene. Supplements may contain higher doses of vitamin C than what can be found in foods. That's because researchers have shown that it helps improve absorption of other nutrients in the body. Taking high doses over long periods can lead to scurvy, though.