Entrainment (chronobiology)
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In the study of
chronobiology Chronobiology is a field of biology that examines Time, timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to sun, solar- and moon, lunar-related rhythms. These cycles are known as biological rh ...
, entrainment occurs when rhythmic physiological or behavioral events match their period to that of an environmental oscillation. It is ultimately the interaction between
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...
s and the environment. A central example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
's rotation. Exposure to certain environmental stimuli will cue a phase shift, and abrupt change in the timing of the rhythm. Entrainment helps organisms maintain an adaptive phase relationship with the environment as well as prevent drifting of a free running rhythm. This stable phase relationship achieved is thought to be the main function 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. 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. The term entrainment is applied because the biological rhythms are
endogenous Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, ...
: the rhythm persists even in the absence of environmental cues because it is not a learned behavior but something that is inherent in organisms. Of the several possible cues, called '' zeitgebers'' (German for 'time-givers', 'synchronizers', 'external timekeepers'), which can contribute to entrainment, light has the largest impact. Units of circadian time (CT) are used to describe entrainment to refer to the relationship between the rhythm and the light signal/pulse. The activity/rest (sleep) cycle 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 The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regul ...
(SCN) of the anterior
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The 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 such as the liver/heart 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 Huygens's
pendulum A pendulum is a weight suspended from a wikt:pivot, pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, Mechanical equilibrium, equilibrium position, it is subject to a restoring force due to gravity that ...
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 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.


See also

*
Crepuscular In zoology, a crepuscular animal is one that is active primarily during the twilight period, being matutinal, vespertine, or both. This is distinguished from diurnal and nocturnal behavior, where an animal is active during the hours of dayli ...
– Animals active at twilight (i.e. dusk and dawn). * Diurnality – Animals active during the day and sleeping at night. *
Nocturnality Nocturnality is an animal behavior 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 highly developed sen ...
– 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