Some comments on the measurement of circadian rhythms after time-zone transitions and during night work

Adjustment of circadian rhythms to changed sleep/wake schedules, as after time-zone transitions and during night work, is not immediate. Different variables appear to adjust at different rates, and such external and internal dissociation is linked with a general malaise ("jet lag") after a time-zone transition. Work using "constant routines" and "purification" methods indicates that the internal dissociation results from differences between variables in the relative contribution of exogenous ("masking") effects to the measured rhythm. These results indicate, therefore, there is no need to postulate the presence of more than one body clock which is responsible for the endogenous component of a circadian rhythm, and that heavily masked circadian rhythms will be poor indicators of the rate of adjustment of this clock. Nevertheless, it will be the measured rhythm that describes most directly the disruption to circadian rhythmicity caused by changed sleep/wake schedules.