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Question: With the exception of sunscreen, why do I have to treat my skin differently from day to night? Can it really tell the difference between AM-and-PM-targeted products?

Answer: Yes—your skin, a complex organ that functions like a body clock with a circadian rhythm which affects dermal functions, is smarter than you might think and its needs differ depending on the time of day. “During the day, skin is in a protective state, warding off elements such as sun damage or oxidation, and at night, your immune system is functioning at its maximum capacity and your skin is going into repair mode, regenerating new skin cells and cycling oxygen and nutrients. During the hours of 11 PM to midnight, the process of cell mitosis of the skin peaks whether you’re awake or not, but sticking to a sleep routine and aiming for the same hours of sleep each night is the best way to regulate the health of your skin,” says Dr. Dennis Gross, a NYC-based dermatologist and founder of Dr. Dennis Gross Skincare. That means that the ingredients you put on your skin in the morning—antioxidants, SPF, hyaluronic acid—should act as shields against factors like pollution, temperature, wind and the sun, while the nighttime ones—retinol, niacinamide, vitamin C—should work to treat any damage caused by those factors. Gross’ top pick for after-hours skin care that helps to prep for the prime cell mitosis hour is a daily combination of alpha and beta hydroxy acids: “They work with different mechanisms to remove the top layer of skin. Beta hydroxy acids break down the bridges/bonds between the cells, while alpha hydroxy acids cause the cells themselves to detach/release. The PH flux, or neutralizing step, is so important because it brings the skin back to normal. It’s the blend of these two acids that I’ve found to be most effective and gentle.”

From: Harper’s BAZAAR US