Deriving a "phase response curve" from adjustment to simulated time zone transitions.

Thirty-eight subjects in groups of two to four have been subjected to simulated time zone transitions of 8 hr eastward, 8 hr westward, or 12 hr; all experiments have taken place in a temporal isolation unit, in which the light intensity during the waking periods was 250-300 lux. Circadian rhythms of rectal temperature have been used as a marker of the process of adjustment, the data being analyzed before (raw) and after "purification." Conventional results have been obtained, but when the data have been treated appropriately, they have produced a relationship between shift of the sleep-activity cycle and shift of the temperature rhythms that shows many characteristics of a phase response curve. Even though the factor or factors causing such adjustment are unknown, the results confirm that changes of a consonant set of "weak" zeitgebers is sufficient for entrainment to a new time zone to occur.