This is something Dr Colette Haydon, founder of Lixir Skin , seconds. "If you are leaving the product on the skin overnight it’s better to only use one active at a time to properly benefit from your skincare regime," she told R29. But if you do want to incorporate the two into your routine, this is how to do it safely. "I would recommend using a retinol every evening for four weeks, letting the skin rest for a few days and then switching to an acid like the Lixir Skin BHA/AHA 10% Night Switch, £20 , for three weeks. If you use one active ingredient for too long it can have a plateau effect so keep your skin interested by switching it up." Her top tip? "You could also use acids in a mask form (R29 recommends Tata Harper's Clarifying Mask, £59 ) while using a retinol night treatment as the acid isn’t being left on the skin. However, I would advise never to use the two together as a leave-on nighttime regime."