Avoid sleeping with wet hair to prevent hair fiber breakage and preserve the balance of the scalp.

noli's read

This recommendation is based on solid biophysical principles, although it has not been directly studied in large-scale randomized clinical trials (RCTs). Structurally, water causes the hair fiber to swell and lifts its protective cuticles, making the hair highly vulnerable to repeated friction against a pillow, as explained by Dr. Timothy Schmidt of the University of Utah (expert opinion). Furthermore, retaining moisture overnight creates a warm and humid microclimate. Specialists at the Cleveland Clinic (expert opinion) note that this environment encourages the growth of natural microorganisms, such as the yeast Malassezia, which can cause dandruff or irritation. While the popular belief associating wet hair with catching a cold is scientifically unproven, Carla Valette's advice is perfectly sound for optimizing hair vitality.

the sources

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