Note that this model was taken from the paper ``Modeling human circadian phase and amplitude resetting'' by R. E. Kronauer, Me. E. Jewett and C. A. Czeisler published in Biological Clocks. Mechanisms & Applications, Proceedings of the International Congress on Chronobiology, Paris, September 1997, Elsevier Science, March 1998.
For more information on human circadian rhythms, biological timing and its applications check out the Biological Timing Tutorial website.