Vitamin C is a daytime antioxidant — its entire purpose is neutralising the free radicals generated by UV radiation and pollution throughout the day. Applied at night when there’s no UV exposure, it delivers no antioxidant benefit. Additionally, Vitamin C (ascorbic acid) and retinol function at different pH levels and can destabilise each other in the same routine. The correct protocol is always Vitamin C in the morning followed by SPF, which together form the most powerful anti-aging pairing in skincare science.

Retinol breaks down when exposed to UV light — applying it in the morning means the active ingredient degrades before it can work. It also significantly increases photosensitivity, so daytime application without perfect SPF coverage would accelerate exactly the UV damage you’re trying to prevent. Retinol is specifically matched to the PM routine because it activates during the cell repair cycle that peaks during sleep. Use 3–4 nights per week, never on the same night as AHA exfoliants, and always follow with morning SPF.

AHAs and BHAs dramatically increase photosensitivity by removing the protective dead cell layer that absorbs some UV radiation. Applied in the morning, freshly exfoliated skin receives full UV impact before your SPF can fully compensate — accelerating the sun damage that causes the very hyperpigmentation you’re using exfoliants to treat. Use them at night, and without fail use SPF the following morning. Never use AHAs and retinol on the same evening — both increase cell turnover and combining them causes barrier disruption without proportionally better results.

Niacinamide has no photosensitivity concern, no UV stability issue, and no timing restriction — it works effectively at any time of day. Most people find the morning most useful for its oil control effect (reducing shine throughout the day), but it’s equally effective at night for barrier repair and dark spot fading. It also pairs exceptionally well with retinol at night — applied after retinol, it soothes any barrier disruption the retinol causes while continuing to work on hyperpigmentation.

Hyaluronic acid has no timing constraints whatsoever — use it in both routines. The critical application detail is to apply it to slightly damp skin immediately after cleansing, before other serums. In a dry climate or in a very dry heated room, be aware that HA needs environmental moisture to draw into the skin — applied to completely dry skin in an arid environment it can paradoxically draw moisture upward from deeper skin layers. A light mist of water before application solves this.

SPF is the last step of every morning routine, applied after moisturiser, and it has no exceptions — every morning, including cloudy days, indoor days, and winter days. Up to 80% of UV radiation penetrates cloud cover. UVA — the aging ray — penetrates glass windows. The entire benefit of your retinol, Vitamin C, and AHA routine is undermined without consistent morning SPF protecting the freshly treated skin from new UV damage. Nothing in skincare science is clearer: SPF is the most impactful anti-aging step available at any price.