Use of Light to Treat Jet Lag: Differential Effects of Normal and Bright Artificial Light on Human Circadian Rhythms

The endogenously generated circadian rhythmicity is modified by special exogenous stimuli. Such an external modification can be manifested in two different modes. First, under constant conditions when the rhythms free-run, the period and many other rhythm parameters depend on the magnitude of the continuously operating effective stimulus; only within a limited range of external conditions (oscillatory range) does the rhythm persist in self-sustaining. Second, under the influence of a periodically operating effective stimulus (zeitgeber), the rhythm is synchronized, but only within a limited range of periods (range of entrainment). It is an experimental result in human circadian rhythms (but not a theoretical necessity) that every stimulus that is effective in one of the two modes mentioned is also effective in the other mode.’ The existence of a phase response curve is not an independent mode of effectiveness of a stimulus, but is an equivalent to the zeitgeber effectiveness of the same stimulus. This means that every stimulus that is effective as a zeitgeber when operating periodically releases a phase shift of the rhythm when given singly in an otherwise constant environment, and that is a phase shift, the amount and direction of which depend on the phase of the rhythm hit by the stimulus. Whereas phase relations under a defined zeitgeber, however, can be measured unambiguously and, hence, the respective phase response curve can be deduced indirectly and unambiguously, the direct measurement of a phase response curve meets with fundamental problems: every stimulus necessary for the measurement of one point within the phase response curve simultaneously alters the phase response curve. All parameters of the phase curve vary with the external conditions; for example, they are different in autonomous and heteronomous rhythms, and they differ even among heteronomous rhythms under the influence of zeitgebers with equal strength but with different periods.” The relative strength of a zeitgeber and, hence, the effectiveness of the stimulus under consideration with regard to circadian rhythms can be measured in two different ways. A first relative measure of the strength of a zeitgeber is the width of the range of entrainment: the larger this range the stronger the zeitgeber (or the weaker is the self-sustainment capacity of the underlying oscillator). Another relative measure of the relative zeitgeber strength is the duration of reentrainment after a phase shift of the zeitgeber: the faster this reentrainment the stronger is the zeitgeber. A phase shift of a synchronizing zeitgeber corresponds to a time shift accompanying a transmeridian flight. Consequently, experiments in which an artificial 24-hour zeitgeber is shifted for several hours simulate long-distance flights over a corresponding number of time zones. With such experiments, therefore, jet lag phenomena can be evaluated.’ In most animal circadian rhythms, light is the most effective external stimulus.