Is the rhythm amplitude related to the ability to phase-shift circadian rhythms of shift-workers?

The magnitude of the circadian acrophase adjustment deltaphi to a phase shift of socio-ecologic synchronizers (as in shift-work) varies from subject to subject. According to J. Aschoff, the ability to adjust rapidly to a phase-shift could be associated with having a small amplitude for certain circadian rhythms. To test this hypothesis, the correlation coefficient (r) between the mean circadian rhythm amplitude A and the acrophase shift deltaphi (measured after the first night-shift) were calculated from estimates of chronobiological time series analyses (Halberg's single cosinor method). Data were obtained from two groups of selected shift-workers (20 and 5 subjects, respectively). A negative correlation between A and deltaphi (the smaller the amplitude, the greater the delpaphi) was observed in the circadian rhythms of the following variables: oral temperature (r = .63;P less than .01), peak expiratory flow (r = . 53: P less than . 01), and urinary 17-OHCS (r = . 60; P less than . 01), but not for other variables such as : grip strength, urinary K+ and Na+. The small amplitude of certain circadian rhythms could be considered as an index of an individual's ability to phase-shift easily. However, chronobiological characteristics, other than the small circadian rhythm ampliture, remain to be identified, for both a better detection of one's ability to do shift-work and a better knowledge for practical applications.