Sebum is the reason your second-day hair often looks better than your freshly washed hair. It’s a mix of triglycerides, wax esters, squalene, and cholesterol produced by sebaceous glands at the base of each hair follicle — chemically, it’s the closest natural product your body makes to a conditioning oil.
On straight hair, sebum travels easily from root to tip, lubricating the whole strand. On curly, coily, or coarse hair, the twists and turns stop the journey — roots get oily while ends stay dry. This is why curly hair benefits disproportionately from conditioners that re-deposit lipids the scalp couldn’t deliver on its own.
When you strip sebum aggressively (with sulfates, for instance), the scalp reads the deficit and ramps up production. Over a few weeks of gentler washing, production normalizes downward — which is why many people find they can wash less often after switching to a sulfate-free shampoo.