On the basis of the description of the circadian system, what is “entrainment,” and how is it related to “synchronization” of circadian rhythms
Circadian rhythms are synchronized to the 24-hour day through the process of entrainment. As outlined above, the SCN is the master autonomous pacemaker for biologic rhythms, but is set at > 24 hours. The synchronization of genetic oscillating systems by a time giver (zeitgeber) is called entrainment. This anchors the internal autonomous clock to external environment. Light is the dominant correcting factor or time cue, capable of inducing sleep phase or wake phase changes. Besides photic stimuli, there are other nonphotic time cues, such as exercise, social interaction, temperature variation, and even feeding; all are capable of shifting circadian rhythms. The interaction between photic and nonphotic clues is complex and the magnitude of contributions to the human system remains to be determined. At this point, it can be said that stable entrainment likely reflects integration of both central and peripheral parameters.