Usually, experts advise that retinol should be applied to cleansed skin in the evening and left on overnight for maximum results. According to Kirsti Shuba, skin specialist and cofounder of Katherine Daniels Cosmetics , our skin cells are typically renewed and repaired at night – the optimum time is 1am, to be precise. But if you're fearful of the effects of sleeping in retinol, Dr. Mahto suggests applying it to your skin for just one hour, instead of a full eight, before rinsing it off. "Think of it like a short contact treatment," she told R29.