Retinol serums should always be used at night and applied to clean skin. Our skin works to rejuvenate itself overnight, so adding in a skin-repairing ingredient like retinol here makes sense.
While a lot of retinol serums are coined as daily use, most skin experts recommend that you start introducing the ingredient slowly. Start with once or twice a week for a couple of weeks, and then build it up slowly as your skin adjusts. Keep an eye on your skin—a little dryness or irritation is normal, but decrease the frequency of your application if you notice anything more than this.
You can go straight to bed after you’ve applied your retinol serum, but if your skin does feel a little tight, then apply a gentle moisturiser afterwards. In fact, if you have sensitive skin, then it’s often recommended that you apply a moisturiser first and your retinol serum on top.