When we are too stressed out from work or going through our personal lives, some people see it. The meaning of this idiom - calm down and "Let your hair down" is to allow yourself to behave freely than usual. Enjoy yourself!
To give us much more reason to be carefree, we must be confident on how we look and feel. Take time to take care of your appearance and take extra attention to your hair. Hair is considered as our crowning glory, hence, it is pertinent that we start with it.