The answer is in the definition. Hygral fatigue= hair getting fatigued from changing between "wet" and "dry". If your hair got wet often enough, the hair cuticle changes.
1. Gummy hair when it’s wet
How to fix it
2. Hair feels limp & weak
3. Hair structure is gone / weakened
You can't "repair" actual hygral fatigue, other than cutting your hair. But there are ways to prevent it from happening in the first place!
A study found that the damage to the hair's cell membrane complex is bigger, the longer it takes for your hair to dry. Find some ways to dry your hair even without heat below.