Which active ingredients should I use in the morning and which at night?

A simple beginner rhythm is antioxidants in the morning and renewing actives at night. Vitamin C (L-ascorbic acid) fits the AM because it adds daytime antioxidant protection alongside — not instead of — sunscreen, while a retinoid such as retinaldehyde suits the PM, partly because retinoids can increase sun sensitivity and are best kept to nighttime. Keeping strong actives on opposite ends of the day (for example vitamin C in the morning and a retinoid at night) also lowers the chance of irritation from layering them together on reactive skin. Whatever you use in the morning, finish with a broad-spectrum sunscreen. This is cosmetic reference for building a routine, not medical advice.