Wet hair is weaker than dry hair. Water breaks hydrogen bonds — the temporary links holding the protein together. Breaking and remaking them does no harm in itself, but while the hair is wet it stretches further and gives way under less force. The cuticle scales also swell unevenly and lift, so the surface turns rough and catches against itself.

So how you dry matters more than what you dry with. Rubbing your head with a towel and blotting it are very different operations. So are pulling a section taut under a round brush and separating it with your fingers. And falling asleep with a wet head, pressing it into the pillow with your whole weight, is also a way of drying, and the slowest and least kind one.

With a dryer, distance decides everything. In a study where strands were washed and dried daily for a month, hair held at 15 cm reached 47 °C and took a minute to dry; at 10 cm it reached 61 °C and took half that; at 5 cm it reached 95 °C and dried in fifteen seconds. Cuticle damage turned up in every blow-dried sample, but the hotter the air, the more of it there was.

There is a separate line at around 90 °C. Above it, the water inside the hair boils faster than it can escape and forces its way out, leaving cavities behind. That is why a straightener on wet hair is one of the worst things you can do — the snapping and crackling you hear is exactly that. A dryer held at a hand's width stays well below that line, because the evaporating water carries the heat away. Held at five centimetres, on the numbers above, it does not.

The idea that air drying is worse than a dryer rests on a single observation in that same study, which nobody has since repeated. It does have a different drawback, though: the hair stays wet and fragile for longer. If you are going to lie on it anyway, dry it first.

There is also an argument the other way. Roots are the last part to dry, and a hat holds them damp for hours after the length feels dry — so they stay weak and rough long after you have stopped thinking of your hair as wet. If you are covering your head, dry the roots first, even if you leave the length to itself.

Blot rather than rub, and don't dry it until it squeaks. The rest is a question of time and convenience.