The reason why nails grow long fast is because of active metabolism of tissues at the nail root. This means that qi movent around fingertips are extremely intense.
When disorders of the body affect movement of qi around fingertips, there appear layers transversely or grooves lengthwise in nails. Such layers represent the severe bad condition during a short period, and longitudinal grooves of especially young people represent chronic ill health. Nails grow from the root to the tip in around six months, so layers of nails will tell you when your body was in a bad condition.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.