Phase shifts of human circadian rhythms due to shifts of artificial Zeitgebers.
In special isolation units, circadian rhythms of human subjects have been investigated under the influence of artificial 24-h Zeitgebers, with 6-h advance and 6-h delay shifts of the Zeitgeber simulating time zone shifts. In most cases, the biological rhythms follow the Zeitgeber shifts in the course of several days: in rare cases, advancing Zeitgeber shifts are followed by delaying shifts of the biological rhythms, either of all variables or, partitioning, of only some of the variables. The rhythm of activity is re-entrained after both Zeitgeber shifts within a few days, independent of the shift direction. The rhythm of rectal temperature needs more time for re-entrainment than the activity rhythm; the rate of re-entrainment is consistently higher after advance than after delay shifts ('direction asymmetry'). Mean value and amplitude of the rectal temperature rhythm are, for some days, reduced after the advance but not after the delay Zeitgeber shift; among the different subjects, the reduction in amplitude is significantly correlated with the direction asymmetry. The rhythm of psychomotor performance (computation speed) re-entrains in parallel to that of rectal temperature; i.e. the performance level is decreased after advance but not after delay shifts. The direction asymmetry in the re-entrainment rates seems to contradict findings in flight experiments where this rate is mostly higher after westward than after eastward flights. Careful considerations, however, show that differences in the re-entrainment behavior after real and simulated time zone shifts disappear when the experimental designs are approximated and when identical procedures of analyzing the data are applied. The results of the time shift experiments are, in all respects tested, in agreement with theoretical postulations; hence, they confirm once more properties of the circadian system deduced earlier. On the other hand, the results are of practical importance since they state significant correlations between the re-entrainment behavior and rhythm parameters measured before the Zeitgeber shifts; this behavior, therefore, can be predicted from data obtained already before the Zeitgeber has been changed in any way: The duration of re-entrainment is correlated with the amplitude, and the decrement in performance with the phase of the rectal temperature rhythm. These practical implications may also apply to shift work.
D000222:Adaptation, Physiological; D000328:Adult; D000337:Aerospace Medicine; D001831:Body Temperature; D002940:Circadian Rhythm; D003463:Cues; D018504:Ecological Systems, Closed; D005260:Female; D006801:Humans; D008297:Male; D008606:Mental Processes / Q000502:physiology; D010561:Personnel Staffing and Scheduling; D012007:Rectum / Q000502:physiology; D012890:Sleep / Q000502:physiology; D013997:Time Factors; D013998:Time Perception