Your liver runs on its own clock. So does your skin. So does your gut. Each organ keeps a semi-independent rhythm, and every one of them takes its cue from a single signal: the light your eyes register in the first hour after you wake up

Before peptides, before retinol, before any serum promising to turn back the clock — there was SPF. It's still the most studied, most proven longevity tool in skincare, and most...