Entrainment (chronobiology)
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In the study of chronobiology, entrainment refers to the synchronization of a biological clock to an environmental cycle. An example is the interaction between
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., Endogeny (biology), endogenous) and responds to the env ...
s and environmental cues, such as
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
and
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
. Entrainment helps organisms adapt their bodily processes according to the timing of a changing environment. For example, entrainment is manifested during travel between time zones, hence why humans experience jet lag. Biological rhythms are endogenous; they persist even in the absence of environmental cues as they are driven by an internal mechanism, most notably the
circadian clock A circadian clock, or circadian oscillator, also known as one’s internal alarm clock is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time. Such a clock's ''in vivo'' period is necessarily almost exact ...
. Of the several possible cues (known as '' zeitgebers,'' German for 'time-givers') that can contribute to entrainment of the circadian clock, light has the greatest impact. Units of circadian time (CT) are used to describe entrainment to refer to the relationship between the rhythm and the light signal/pulse.


Modes of entrainment

There are two general modes of entrainment: phasic and continuous. The phasic mode is when there is limited interaction with the environment to "reset" the clock every day by the amount equal to the "error", which is the difference between the environmental cycle and the organism's circadian rhythm. Exposure to certain environmental stimuli will cause a phase shift, an abrupt change in the timing of the rhythm. The continuous mode is when the circadian rhythm is continuously adjusted by the environment, usually by constant light. Two properties, the free-running period of an organism, and the phase response curve, are the main pieces of information needed to investigate individual entrainment. There are also limits to entrainment. Although there may be individual differences in this limit, most organisms have a +/- 3 hours limit of entrainment. Due to this limit, it may take several days for re-entrainment.


Mechanisms of entrainment

The activity/rest cycle (sleep) in animals is one of the circadian rhythms that normally are entrained by environmental cues. In mammals, such endogenous rhythms are generated by the suprachiasmatic nucleus (SCN) of the anterior
hypothalamus The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...
. 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 such as the
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
/
heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...
as peripheral oscillators, and it is believed that they sync up with the master pacemaker in the mammalian brain, the SCN. 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 pendulum clocks.


Health implications

When good sleep hygiene is insufficient, a person's lack of synchronization to night and day can have health consequences. There is some 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.


Applications of entrainment

Entrainment is used in various fields to optimize performance and health. In sports, it helps athletes adjust to new time zones quickly. In medicine, light therapy is used to treat circadian rhythm disorders. The principles of entrainment are also applied in occupational health to design better shift work schedules.


See also

*
Crepuscular In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal (active during dawn), vespertine (biology), vespertine/vespertinal (active during dusk), or both. This is distinguished from diurnalit ...
– Animals active at twilight (i.e., dusk and dawn). *
Diurnality Diurnality is a form of plant and ethology, animal behavior characterized by activity during daytime, with a period of sleeping or other inactivity at night. The common adjective used for daytime activity is "diurnal". The timing of activity by ...
– Animals active during the day and sleeping at night. *
Nocturnality Nocturnality is a behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite. Nocturnal creatures generally have hi ...
– Animal activity of sleeping during the day and active at night.


References

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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. Circadian rhythm