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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 rhythms and environmental cues, such as light and temperature. Entrainment helps organisms adapt their bodily processes with the timing of a changing environment.^[1] 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, the circadian clock being the best characterized. Of the several possible cues, known as zeitgebers (German for 'time-givers'), which can contribute to entrainment of the circadian clock, light has the greatest impact.^[2]^[3] Units of circadian time (CT) are used to describe entrainment to refer to the relationship between the rhythm and the light signal/pulse.^[4]

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.^[5] Due to this limit, it may take several days for re-entrainment.^[6]

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. Entrainment is accomplished by altering the concentration of clock components through altered gene expression and protein stability.^[7]

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 pendulum clocks.^[8]

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.^[9]

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.^[10] The principles of entrainment are also applied in occupational health to design better shift work schedules.

References

^ Olds, William (2015). Sleep, Circadian Rhythms, and Metabolism: The Rhythm of Life. Apple Academic Press. ISBN 978-1771880626.^{[page needed]}
^ Edgar, DM; Dement, WC (1991). "Regularly scheduled voluntary exercise synchronizes the mouse circadian clock". American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 261 (4): R928–R933. doi:10.1152/ajpregu.1991.261.4.R928. PMID 1928438.
^ Van Reeth, O; Sturis, J; Byrne, MM. "Nocturnal exercise phase delays circadian rhythms of melatonin and thyrotropin secretion in normal men". American Journal of Physiology-Endocrinology and Metabolism. 266 (6): E964–E974.
^ Pittendrigh, CS (1981). "Circadian Systems: Entrainment". Handbook Behavioral Neurobiology. 4: 239–268.
^ Klein, DC (1991). Circadian Rhythms: The Molecular and Neuroanatomical Basis of Biological Timing. MIT Press. ISBN 9780262111628. {{cite book}}: Check |isbn= value: checksum (help)
^ Refinetti, Roberto (2006). Circadian Physiology. Taylor & Francis. ISBN 9780849322334.^{[page needed]}
^ Toh, Kong Leong (August 2008). "Basic Science Review on Circadian Rhythm Biology and Circadian Sleep Disorders" (PDF). Annals of the Academy of Medicine, Singapore. 37 (8): 662–8. doi:10.47102/annals-acadmedsg.V37N8p662. PMID 18797559. S2CID 11071556. Archived from the original (PDF) on 2009-10-07. Retrieved 2009-08-15.
^ Yoo, SH; Yamazaki, S (2004). "PERIOD2::LUCIFERASE Real-Time Reporting of Circadian Dynamics Reveals Persistent Circadian Oscillations in Mouse Peripheral Tissues". Proceedings of the National Academy of Sciences. 101 (15): 5339–5346. Bibcode:2004PNAS..101.5339Y. doi:10.1073/pnas.0308709101. PMC 397382. PMID 14963227.
^ "Circadian Rhythm Sleep Disorders". National Institute of General Medical Sciences. Retrieved 2024-07-30.
^ Lewy, AJ (2000). Light Therapy and Non-Solar Radiation. Oxford University Press. ISBN 9780195131064. {{cite book}}: Check |isbn= value: checksum (help)