Human body is sustained by incessant flow of matter, such as flow of blood, electric substances, atoms and etc. These are summed as flow of qi.
One direction of such flow is up and down flow. When a person is tired, there occurs stagnation of qi in the head. Generally speaking, women prevent stagnation of qi by not letting their bodies get cold, men prevent by emanating qi. This is why women have more hair and men have less.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.