Re-entrainment of circadian rhythms after phase-shifts of the Zeitgeber.

To describe anabrupt shift of the Zeitgeber sufficiently, it is necessary to indicate whether the phase has been advanced or delayed, for how many hours it has been shifted, and what part of the Zeitgeber has been lengthened or shortened respectively. The entrained circadian system usually follows a shift of the Zeitgeber in the same direction and by an equal amount. However, there can be exceptions to both of these rules: a) an instantaneous 12-h shift of a (symmertric) Zeitgeber always represents a delya (= doubling of one-half of its period). In contrast to this, the gradual re-entrainment of the circadian system can be accompalished by either advances or by delays (Fig. 2), it can occur in both dirctions simultaneously by splitting the rhythmic variable (Fig. 1),or it can represent an apparent advance not distinghushable from a delay ( Fig. 6); b) after complietion of re-entraiment, the phase-angle difference psi between organism and Zeitgeber an differ systematically from the pre-shift psi-value...