ABSTRACT
Resetting is basic to the functioning of the biological clock. At most latitudes, organisms face a change in Zeitgeber timing with each new day's dawning because the photoperiod has changed. A tool that has been used to study resetting has been the measurement of phase shifts following single short pulses of a Zeitgeber. When scientists methodically exposed organisms to single perturbing signals at different times over 24 hours, the phase shifts they obtained were not all the same. They graphed the direction and amount of phase shift obtained versus the time that the pulse was given. Such a graph is called a phase response curve. One of the reasons that phase response curves capture the interests of circadian biologists is that they offered elegant explanations of fundamental observations of the properties of circadian rhythms, such as the effects of constant conditions on period length and the mechanism of entrainment.
