Why Can I Only Use Tretinoin At Night? by Musely Editors Why Can I Only Use Tretinoin At Night?Tretinoin is a superstar anti-aging ingredient—but only if you use it properly. To make sure you're using tretinoin the right way, it's recommended to be applied at night only (right before you sleep). This is because tretinoin oxidizes when exposed to artificial light or sunlight, which can reduce efficacy.In addition, due to the delicate new skin that forms when you use tretinoin, the complexion is more vulnerable to the sun. Don't think that using tretinoin and SPF during the day will matter either, as tretinoin can still make the skin susceptible to sun damage even when an SPF is worn. Tretinoin is a fabulous ingredient for the complexion when used properly. Remember the simple rule to only apply your tretinoin at night, before heading to bed. Avoid daytime use of tretinoin as it can create dryness, redness, and irritation—and no one wants that!