Entrainment (chronobiology)
Entrainment, within the study of chronobiology
Chronobiology is a field of biology that examines periodic phenomena in living organisms and their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobiology comes from the ancient Greek χρόνος , and biology, which pertains to the study, or science,...

, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s 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
Endogenous substances are those that originate from within an organism, tissue, or cell. Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates...

: They persist when the organism is isolated from periodic environmental cues. Of the several possible cues, called zeitgeber
Zeitgeber is any exogenous cue that synchronizes an organism's endogenous time-keeping system to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light...

(German for time-givers, synchronizers), which can contribute to entrainment, bright light is by far the most effective.

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
Suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms...

 of the anterior hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

. Entrainment is accomplished by altering the concentration of clock components through altered gene expression and protein stability.

Circadian oscillations occur even in isolated organs, and it is believed that the master pacemaker in the mammalian brain, the SCN, entrains the periphery. 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
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...


The phase of entrainment refers to the relative timing of any circadian event within the objective 24-hour day.

When good sleep hygiene
Sleep hygiene
Sleep hygiene can be defined as the controlling of "all behavioural and environmental factors that precede sleep and may interfere with sleep." It is the practice of following guidelines in an attempt to ensure more restful, effective sleep which can promote daytime alertness and help treat or...

 is insufficient, a person's lack of synchronization to night and day can have health consequences. There is significant variation within normal chronotype
Chronotype is an attribute of animals, including human beings, reflecting at what time of the day their physical functions are active, change or reach a certain level...

s' entrainment; it is normal for humans to awaken anywhere from about 5 a.m. to 9 a.m. However, patients with DSPS
Delayed sleep phase syndrome
Delayed sleep-phase syndrome , also known as delayed sleep-phase disorder or delayed sleep-phase type , is a circadian rhythm sleep disorder, a chronic disorder of the timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily rhythms, compared to the...

Advanced sleep phase syndrome
Advanced sleep phase syndrome , also known as the advanced sleep-phase type of circadian rhythm sleep disorder, is a condition in which patients feel very sleepy and go to bed early in the evening and wake up very early in the morning Advanced sleep phase syndrome (ASPS), also known as the...

 and Non-24-hour sleep-wake syndrome
Non-24-hour sleep-wake syndrome
Non-24-hour sleep-wake syndrome is a chronic circadian rhythm sleep disorder, classified within Chapter VI, Diseases of the Nervous System, in the ICD-10. It can be defined as "a chronic steady pattern comprising one- to two-hour daily delays in sleep onset and wake times in an individual living...

are improperly entrained to light/dark.

Further reading

  • Pittendrigh CS (1981) Circadian systems: Entrainment. In Handbook Behavioral Neurobiology, Vol. 4. Biological Rhythms, J. Aschoff, ed. pp. 239–68, University of California Press, New York.
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