Is there more than one circadian clock in humans? Evidence from fractional desynchronization studies.

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Abstract
Three groups of 4 healthy volunteers lived in an isolation unit for 24 days. During this time they lived by a clock which, unknown to themselves, ran progressively faste; so, by real time, the day decreased from 24.0 to 22.0 h in length. Throughout this protocol, the subjects lived a regular regimen of sleep, waking and meals based upon their local time clock. They collected regular urine samples that were analyzed for a variety of constituents. Rectal temperature was also recorded automatically throughout. The effects of such a protocol upon circadian rhythmicity in these variables were investigated by a variety of techniques, including cosinor analysis. The temperature rhythm was less able to adjust to a shortening day than were the urinary variables, with the possible exception of K; i.e., the protocol forced an internal desynchronization to exist between different variables. The possibility that more than 1 internal circadian clock might exist, and the direct effect that sleep exerts upon the expression of circadian rhythms, was discussed.