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

Three groups of four healthy volunteers lived in an isolation unit for 24 days. During this time they lived by a clock which, unknown to themselves, ran progressively faster so that, 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 analysed 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 results showed that the temperature rhythm was less able to adjust to a shortening 'day' than were the urinary variables, with the possible exception of potassium; that is, the protocol forced an internal desynchronization to exist between different variables. These results are discussed in terms of both the possibility that more than one internal circadian clock might exist and the direct effect that sleep exerts upon the expression of circadian rhythms.