Fig. 5.

Brain areas reentrain at different rates after a shift in the light schedule. Relative to tissues taken from unshifted animals (filled squares), tissues taken from animals that experienced a 6 hr delay (filled circles) or advance (open circles) took many days to resume their normal phase relationship to the light cycle. Plotted against time since the last light onset of the control group, rhythmicity in the arcuate nucleus (A) peaked 3 hr later than usual after 1 d in the delayed light cycle and appeared close to entrained after 3 d in the new schedule (numbers in parentheses indicate rhythmic cultures per number of cultures tested, and arrowsindicate the point of complete phase shift). After a 6 hr advance, the AN assumed its normal phase after 6 d in the novel light cycle. In contrast, the PVN (B) and pineal (C) appeared to reentrain more rapidly to the advance but took much longer to synchronize to the delayed schedule.