Dr Simon Archer, University of Surrey:
It now appears that virtually all cells in the body have their own ticking circadian clock, including skin cells
Research in the Proceedings of the National Academy of Sciences shows that skin cells can be used to measure the speed of a person's body clock.
Human beings exhibit wide variation in their timing of daily behavior. We and others have suggested previously that such differences might arise because of alterations in the period length of the endogenous human circadian oscillator. Using dermal fibroblast cells from skin biopsies of 28 subjects of early and late chronotype (11 "larks" and 17 "owls"), we have studied the circadian period lengths of these two groups, as well as their ability to phase-shift and entrain to environmental and chemical signals. We find not only period length differences between the two classes, but also significant changes in the amplitude and phase-shifting properties of the circadian oscillator among individuals with identical "normal" period lengths. Mathematical modeling shows that these alterations could also account for the extreme behavioral phenotypes of these subjects. We conclude that human chronotype may be influenced not only by the period length of the circadian oscillator, but also by cellular components that affect its amplitude and phase. In many instances, these changes can be studied at the molecular level in primary dermal cells.
Some models I tried have beautiful 24 hour cycle in them, but you can change reaction coefficients and see how it affects the period of model oscillations. It is very interesting to see how random changes of parameters by order of magnitude quite often do not break the oscillating character of the model.