Some persons will only use products containing Vitamin A at night because the skin turnover is up to 8 times greater during the regenerative hours of sleep and Vitamin A is required for a healthy process of epithelial development and physiologically healthy exfoliation. However, it is not a requirement to only use the product at night. Considering that most skin inflammatory events and free radical hits occur during the daytime, it is overall best to use Pro-Heal Serum Advance+ during the day. However, AM or PM use are both acceptable.