Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by the Earth's rotation. The term entrainment is justified because the biological rhythms are endogenous: They persist when the organism is isolated from periodic environmental cues. Of the several possible cues, called zeitgebers (German for 'time-givers', 'synchronizers'), which can contribute to entrainment, bright light is by far the most effective. Exercise may also play a role.
The activity/rest (sleep) cycle in animals is only one set of circadian rhythms that normally are entrained by environmental cues. In mammals, such endogenous rhythms are generated by the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Entrainment is accomplished by altering the concentration of clock components through altered gene expression and protein stability.
Circadian oscillations occur even in the cells of isolated organs, and it is believed that the master pacemaker in the mammalian brain, the SCN, coordinates the peripheral clocks. Such hierarchical relationships are not the only ones possible: Two or more oscillators may couple in order to assume the same period without either being dominant over the other(s). This situation is analogous to Huygens' pendulum clocks.
The phase of entrainment refers to the relative timing of any circadian event within the objective 24-hour day.
When good sleep hygiene is insufficient, a person's lack of synchronization to night and day can have health consequences. There is significant variation within normal chronotypes' entrainment; it is normal for humans to awaken anywhere from about 5 a.m. to 9 a.m. However, patients with DSPD, ASPD and non-24-hour sleep–wake disorder are improperly entrained to light/dark.