How often should you wash your hair?

Does washing more often make your scalp oilier?

The controlled evidence does not support it.

The rebound theory assumes shampooing signals the sebaceous glands to overproduce. Sebaceous output is largely driven by hormones and genetics, so removing surface oil does not push baseline production up over the long run. What frequency actually changes is how much accumulated, oxidised oil and debris is sitting on the scalp at any given time.

That distinction matters because oxidised sebum is the irritating part. As sebum sits, it oxidises, and oxidised sebaceous lipids drive itch, flaking, and odour. Regular cleansing clears that load before it builds. The study authors concluded that "overcleaning" concerns were unfounded, both objectively and subjectively, in the population they studied.

What did the wash frequency study actually measure?

Two linked studies, more than 1,500 participants, and endpoints beyond self report.

The epidemiological arm surveyed wash habits against measured scalp and hair outcomes. Satisfaction peaked and scalp flaking was lowest at five to six washes per week. The controlled arm compared daily washing against a seven day refrain from washing. Daily washing outperformed weekly on every objective and self reported endpoint measured: scalp flaking, itch, dryness, odour, oxidised sebum lipids, and self reported good hair days.

The result most people expect to go the other way did not. Hair loss, hair brittleness, and hair smoothness showed no adverse effect from frequent washing, and slightly improved.

Two honesty notes belong with those numbers. The population was Asian adults, predominantly straight haired, without scalp pathology. And one author affiliation is industry, from a shampoo brand lineage, which is common in cosmetic science wash studies and means a pro washing result aligns with commercial interest. Neither invalidates the findings. Both should be visible when the findings are quoted.

Why does "wash once a week" fail as a rule?

Because it fixes a number for a variable that differs per person, and it does so in the wrong direction for most scalps.

Sebum rebuilds within a day or two. On a weekly schedule, five or six of those days are spent with sebum accumulating and oxidising on the scalp. For a normal to oily scalp that is not a neutral choice, it is the mechanism behind flaking and itch running unchecked.

The rule also survives on a misdiagnosis. People who wash rarely with a harsh shampoo do experience stripping and rebound feeling oiliness, and they correctly notice something is wrong. The problem is the harshness, not the frequency. Cutting frequency treats the symptom and leaves the cause running.

Does this apply to curly and coily hair?

Not directly, and it should not be stretched to.

The study population was straight haired adults without scalp disease. Sebum travels down a straight fiber readily and down a tightly curled one poorly, so the same scalp cleansing schedule does not produce the same result along the length of the strand. Lower frequency is standard advice for type 3 and type 4 hair for that reason, and nothing in this study overturns it.

What the study does support, across the board, is that frequent washing is not inherently damaging. Take that as the removal of a fear, not as the arrival of a new number.

So what actually decides your number?

Your scalp, your sweat, and the shampoo you are using.

An oily scalp, a scalp that flakes or itches, or heavy training and sweating all push frequency up. A dry or reactive scalp pushes it down. Tighter curl patterns push it down. And frequency does not get decided in isolation from formula: frequent washing with a gentle, mildly acidic shampoo is a different proposition than frequent washing with an aggressive one. The pairing is the answer, not either half alone.