Resynchronization of the circadian system following a 9-hr advance or a delay zeitgeber shift: real flights and simulations by a Van-der-Pol oscillator.

After a flight from Germany to California, eight subjects stayed there for 3 weeks and then were flown back to Germany. After both the westbound and the eastbound flights, body temperature was measured for 2 weeks in the new time zone. Data obtained every 3 hr throughout the day and night were analyzed by the complex demodulation technique to visualize phase adjustments and the nonstationary amplitude of the measured circadian rhythms. The response to the zeitgeber delay was very similar in all subjects and consisted of a phase adjustment of 6 hr in the first 2 days following the flight and a subsequent slow completion of the resynchronization with a rate of 0.5 to 1 hr per day. After the eastbound flight the resynchronization was generally slower, and the circadian amplitudes were more reduced than after the zeitgeber delay. In addition, the advance shift led to different patterns of phase adjustment for the individuals. Shortening and lengthening of circadian periods as well as phase jumps by about 12 hr were observed. Simulations with a Van-der-Pol oscillator forced by a rectangular zeitgeber function clearly reflected the divergence in resynchronization between eastbound and westbound flights. The different resynchronization patterns found after the 9-hr zeitgeber advance, including the corresponding duration of readjustment, could also be well simulated.