Is an eastbound circumpolar flight accompanied by phase advance of biological rhythms?

In the circumpolar region, the light-dark environment, which is the most potent Zeitgeber among others, and to which polar route travelers are exposed, varies greatly with the season and the time of day. Hence, resynchronization following an eastbound flight through this region will not necessarily be attained by phase advance of the biological rhythms. To prove this by simulation, a computer program was produced so as to locate the sun at any moment on any day and at any point on the earth. Rats were exposed to the changing pattern of light-dark condition which simulated flight schedules of polar, great circle and southern routes. Observation was made on the progress of biological resynchronization to the local time. The polar route simulation with an advance of 15 hours followed by a delay of the same hours resulted in phase delays on both trips. The great circle route simulation with a shift of 9 hours back and forth gave the same results. Another polar route simulation with an advance of 18 hours also resulted in a delay, while on their way back home some showed delay and some did advance. These findings suggest that the light-dark pattern after arrival works more rather than that during the trip, and that the resynchronization is apt to be achieved by delaying rather than by advancing, particularly when advancing of the phase by 6 hours or more is required. The critical limit seems to be subject to difference in species.