Orentreich N, Markofsky J, Vogelman JH

J. Invest. Dermatol. 1979 Jul;73(1):126-30

PMID: 448171

The linear nail growth rate is a simple, inexpensive, noninvasive technique for the measurement of aging. When the many endogenous and exogenous factors known to influence this rate are either controlled or considered, the measurement of the rate for 1 year gives data that are both age-related and age-caused. The rate of linear nail growth decreases 50% over the life spans of both dogs and humans. In the beagle, which has a life span 20% that of man, the rate of decrease is 5 times faster than in man. There are circadian and multiple-year biorhythms of the rate of linear nail growth. There are approximately 7-yr periods of slow decline that alternate with 7-yr periods of more rapid decline.

http://www.ncbi.nlm.nih.gov/pubmed/448171