2. Apply retinoids only at night; they are sensitive to sunlight and could lose potency if applied in the morning.
3. AHAs might deactivate retinoids; if you use them both, apply the AHA in the morning and the retinoid before bed (or, if your skin is sensitive, on alternating days).
4. For the most effective protection against skin-damaging free radicals, use a mix of antioxidants—morning and night.
5. Apply all treatments right down to your collarbone—except retinoids, which can irritate the thin skin on your neck. Bonus points if you include your hands.
6. Unless the stars are out, wear sunscreen. And reapply often.
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