Circadian rhythms: influences of internal and external factors on the period measured in constant conditions.

The article reviews most of the data available on the period tau of freerunning circadian rhythms, measured in constant conditions. Emphasis is placed on the effects of light intensity and ambient temperature on tau, with references to influences of other external as well as internal factors. In the introduction, examples are given of spontaneous and induced variations in tau and its dependence on the experimental history. The discussion concentrates largely on results obtained from arthropods and vertebrates.