Hair is dead; the scalp is alive. Everything washing does, it does to the scalp. A yeast called Malassezia lives on everyone and feeds on your sebum, cracking it into free fatty acids that inflame the skin, push out flakes, and drive the itch most people feel by day two or three ^{1 2}. Washing clears that load before it turns inflammatory.

The rebound myth has a number on it now. Two studies on roughly 1,500 adults found scalp and satisfaction best at 5–6 washes a week; the controlled arm switching from weekly to daily got lower sebum, fewer flakes, less odor ³. The enemy isn't frequency, it's harshness: daily sodium lauryl sulfate down the lengths of treated hair is what actually wrecks it. And shampoo doesn't cause hair loss — the strands in the drain were already shedding.

Match the cadence to your hair type — the single biggest lever here ⁴.

The second-day itch goes first. Within a week or two on the right cadence, the day-one-fine, day-two-scratchy pattern flattens out. If you were over-washing, the lengths stop feeling like straw in a few weeks and split ends slow down. Over months, color holds longer and the trim cycle stretches. Nothing dramatic — just hair behaving instead of slowly going wrong.

If you have visible flakes, that's usually Malassezia, not buildup, and a medicated antifungal shampoo earns its place: zinc pyrithione, ketoconazole, or selenium sulfide, two or three times a week. Lather and leave it on the scalp 5–10 minutes before rinsing; a 30-second wash gives the active no time to work. Once it clears, taper to twice weekly; stopping entirely usually brings it back ⁵.