In circadian rhythms, the operation of the endogenous oscillator sets a response to occur at a particular time of day. A single oscillator can be coupled to multiple circadian rhythms, which may even be out of phase with each other. How do such responses remain on time when the daily durations of light and darkness change with the seasons? The answer to this question lies in the fact that the phase of the rhythm can be changed if the whole cycle is moved forward or backward in time without its period being altered.
Investigators test the response of the endogenous oscillator usually by placing the organism in continuous darkness and examining the response to a short pulse of light (usually less than 1 hour) given at different phase points in the free-running rhythm. When an organism is entrained to a cycle of 12 hours light and 12 hours dark and then allowed to free-run in darkness, the phase of the rhythm that coincides with the light period of the previous entraining cycle is called the subjective day, and the phase that coincides with the dark period is called the subjective night.
If a light pulse is given during the first few hours of the subjective night, the rhythm is delayed; the organism interprets the light pulse as the end of the previous day (see Figure 24.15D). In contrast, a light pulse given toward the end of the subjective night advances the phase of the rhythm; now the organism interprets the light pulse as the beginning of the following day.
As already pointed out, this is precisely the pattern of response that would be expected if the rhythm is to stay on local time. Therefore, these phase-shifting responses enable the rhythm to be entrained to approximately 24-hour cycles with different durations of light and darkness, and they demonstrate that the rhythm will run differently under different natural conditions of day length.
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