The most of people experience a cramp in the leg when they get old to some extent. It is understandable to get a cramp after the hard exercise and so on, but sometimes one is seized with a cramp and wakes up during sleep at night.
This can be understood as the fatigue of the body itself appearing in the legs where the pressure was put the most. In other words, the cramp is not only the problem of the legs. The fatigue is what the movement of qi of the body gets slow and cold, so needless to say when using one's legs, and even when yang qi gets cold at night, the qi movement of legs slows down. When it gets stronger, the pain appears in the legs. To prevent a cramp, it is important to think about the cause of the fatigue.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.