To maintain healthy skin, you must seal in your treatment serums with a moisturizer every night and/or sunscreen during the day. Water can and will evaporate through the skin. This process is called transepidermal water loss or TEWL. Moisturizers help prevent TEWL. Moisturizers also help protect your skin against environmental and free radical attack. You may require different moisturizers for day use and night use or for different times of year.