Adjust the water temperature when washing your hair (choosing between hot and cold water) to preserve shine and protect the hair fiber.

noli's read

The idea that water temperature must be adjusted is based on a persistent popular belief, but hair science strongly qualifies these benefits. On one hand, beauty experts agree that one should avoid very hot water, as it strips natural oils from the scalp, leading to dryness and frizz. On the other hand, the myth that cold water closes cuticles to boost shine is contradicted by concrete data. An experimental study conducted by the research institute TRI Princeton demonstrated that rinsing hair at less than 18°C provided no additional luster compared to lukewarm water, with the latter even removing product residue more effectively. Biologically, the external hair fiber is a dead structure (keratin) and does not possess a thermal mechanism to actively open or close its scales. Finally, specialists at the Philip Kingsley trichology clinic (expert opinion) suggest that ice-cold water can temporarily contract the blood vessels of the scalp, anecdotally limiting nutrient supply without proof of actual harm to hair growth.

the sources

No named source held this one up — that absence is part of the verdict.