Numerous organisms are equipped with circadian clocks (internal body clocks) that help them adapt to daily environmental changes in light and temperature. For synchronizing the clock phase (timing) with these environmental rhythms, the circadian clock alters its phase in response to synchronizing stimuli such as light and temperature. This is known as a phase response. It is important to note that the magnitude of this response varies, depending on the phase of the stimulus received.
Click here for original story, A simplified method to assess the synchronization properties of the body clock
Source: Phys.org