Circadian adaptation to night shift work

By Betty Simmons,2015-01-15 03:14
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Circadian adaptation to night shift work

    Stručno-znanstveni skup


    27. rujna 29. rujna 2006

    Bjelolasica, Hrvatska



     123M. Martinis, V. Mikuta-Martinis, Lj. Škovrlj 1 Institut Ruđer Bošković, Zagreb, HRVATSKA, e-mail: 2 Institut Ruđer Bošković, Zagreb, HRVATSKA, e-mail: 3 Tekstilno-tehnološki fakultet, Zagreb, HRVATSKA, e-mail:

    Sažetak: Cirkadijanski ritmovi radnika u noćnoj smjeni ne prilagođuju se odmah na neprirodnu rasporedjelu rada i spavanja. Rad u noćnim smjenama smanjuje kvalitetu života i izaziva često puta opasne zdrastvene i sigurnosne probleme. Poremećaji cirkadijanskih ritmova mogu rezultirati višestrukim negativnim simptomima kao što su loš rad i i smanjena pozornost za vrijeme noćnog rada kao i loš dnevni san kod kuće. Ovaj članak daje prikaz zdrastvenih i sigurnosnih problema

    vezanih za (ne)mogućnost prilagodbe cirkadijanskog ritma rasporedu noćnog rada i dnevnog spavanja. Prvo ćemo opisati osnovna načela cirkadijanskih ritmova kod ljudi koji su odgovorni za prilagodbu smjenskom radu. Ova načela omogućuju iznalaženje odgovarajučeg rasporeda rad/spavanje – dan/noć koji bi ubrzao cirkadijansku prilagodbu noćnom radu. U

    zaključku ovog prikaza spomenuti ćemo ulogu osobnosti pojedinca na moguću cirkadijansku prilagodbu.

    Ključne riječi:smjenski rad, cirkadijanski ritmovi, poremećaj sna, prilagodba

    Summary: The circadian rhythms of night shift workers do not usually phase shift immediately to adapt to their unusual work and sleep schedules. The night shift work reduces workers quality of life and produces potentially dangerous health and safety problems. The circadian rhythms misalignments result in a multitude of negative symptoms including poor work performance and reduced alertness during night work and poor daytime sleep at home This paper reviews these health and safety problems connected with circadian adaptation ability, ie the shifting of circadian rhythms to align with night work and day sleep schedules. We first describe basic principles of human circadian rhythms relevant to producing circadian adaptation to night shift work.. These principles should enable designing appropriate work/sleep-light/dark schedules for accelerating circadian adaptation in night shift workers. . We conclude this review by discussing the impact of individual differences on possible circadian adaptation.

    Keywords: Shift work, Circadian rhythms,Sleep disorder, Adaptation


    Shift work has become a part of our society. The number of shift workers is constantly rising keeping pace with the trend in the consumer demand to acces goods, services, and technological support 24 hours a day. Although the shift work does have benefit for the employers in terms of production, research evidence exists indicating the negative physical, psysiological, and psychosocial effets of working a shift schedule. Medical and sociological research documents some of the potential costs of shift work: disrupted biological rhyrhms, insufficient sleep, poor eating habits contributing to emotional and/or physical health problems. Shift work is associated also with cardiovascular disease, gastrointestinal disease, increased accident risk, disturbed sleep and increased fatigue.

    The sleep - wake and rest - activity cycles in humans are controlled by an endogenous circadian (~24-hour) clock. This clock also drives rhythmic fluctuations of many other aspects of human physiology, such as body temperature, blood pressure, heart rate and the release of various endocrine hormones. Since they are controlled by an internal master clock, these processes continue to cycle even when people are removed from the cyclic day - night environment. However, any kind of desynchrony between the endogenous master clock and the environment, as is caused by travel to a different time zone or by night shift work, results in a multitude of

    physiological disturbances [6].

    The unhealthy symptoms and many harmful consequences to mind and body of shift work can be explained by a temporal mismatch between the work-sleep schedule and the internal circadian rhythms. This mismatch occurs because the 24-h external zeitgebers, such as the natural light-dark cycle, keep the circadian rhythms from phase shifting to align with the night-work, day-sleep schedule. Knowing an individual's circadian phase is important for designing treatments for adaptation to shift work, and in the diagnosis and treatment of various circadian rhythm sleep disorders [1]. Standard, rather invasive, method to measure circadian rhythm phase is the body temperature or fluctuations in hormonal levels [7].



    Biological rhythms control much of the body's normal functions, including activity, behavior, sleep and endocrine rhythms. The circadian rhythm is the reason that we feel like getting up in the morning and what causes us to feel sleepy at bed time. These functions are primarily regulated by the circadian master clock, a

    cluster of approximately ten thousand nerve cells located on the suprachiasmatic nuclei (SCN) found on the hypothalamus in the brain. The circadian clock's primary task is to interpret external changes of light and darkness, as well as social contact, in order to establish diurnal rhythms. It is not uncommon for the circadian master clock to be disrupted temporarily by events such as changes in work schedule from day to night. Secondary circadian oscillators are found in every organ and, indeed, in every cell. Moreover, each organ has its own appropriate zeitgeber to synchronise these clocks. Although light is the major zeitgeber for the SCN, it does not affect clocks in the liver, for example. The zeitgeber for the liver is food, but food is not a zeitgeber for the SCN. These two clocks have a phase difference.

    Abrupt shifts in everyday routine, such as night shift work, or travel can alter the sleep cycle and have a harmful effect on normal circadian rhythms.. If the alterations are strong enough and long, they may induce mood disorders including mild depression and other affective disorders.

    Circadian rhythms are endogenous rhythms that cycle on a daily (approximately 24 hour) basis under normal circumstances. The circadian cycle regulates changes in performance, endocrine rhythms, behavior and sleep timing More specifically these physiological and behavioral rhythms control the waking/sleep cycle, body temperature, blood pressure, reaction time, levels of alertness, patterns of hormone secretion, and digestive functions.


    Two specific forms of circadian rhythms commonly discussed in research are morning and evening types. People considered morning people rise between 5 a.m. and 7 a.m. go to bed between 9 p.m. and 11 p.m., whereas evening people tend to wake up between 9 a.m. and 11 a.m. and retire between 11 p.m. and 3 a.m. The majority of people fall somewhere between the two types. Evidence has shown that morning types have more rigid circadian cycles then evening types, who display more flexibility in adjusting to new schedules. One theory says that evening types depend less on light stimuli from the environment to shape their sleep/wake cycle, and therefore exhibit more internal control over their circadian rhythms [4].


In night shift work, the clock genes in the brain gradually adapt to a phase shift of the lightdark cycle, whereas

    clock genes in the muscle, liver and lung resynchronise at their own rates. This results in a ‘double

    desynchronisation’–‘internal desynchronisation’ between different clocks in the body and brain, and ‘external

    desynchronisation’ between the timing of body rhythms with respect to the lightdark cycle. The temporal

    ordering can get quickly out of tune. The problems of a shift worker who is awake, active and eating main meals at night, although he receives daylight on the way home, are obviously clear [2]. Complete adaptation to night work is almost impossible, and the desynchronised status can rarely be overcome. This misalignment has profound effects on mood, sleep and health.


    Light is an important factor for maintaining biological rhythms. The circadian clock relies heavily on changes in light to determine transitions from night to day. During periods of darkness the master clock sends out the hormone melatonin, which induces sleep. It is easy to see how changing work schedules from the day shift to the night shift would create the need to reverse this process, which takes time and will in turn disrupt normal rhythmic patterns. The shift workers gradually alter their sleep/wake patterns after a change in working hours.

     Their circadian rhythms were shown to adjust an hour or two per day It could take over one week for an individual to fully adjust to an 8-hour shift change.



    Ultradian rhythms (less than a day cycles) regulate short-term patterns, such as feeding cycles. It has been shown that behavior related to feeding is indeed caused by a biological rhythm and not external environmental stimuli. Although circadian master clock, located in brain, is not responsible for regulating feeding intervals, the studies conducted on rats showed that rats on a feeding cycle between 22 and 26 hours (the circadian range) where better able to anticipate food arrival. It can be shown that a drastic change in meal times would have a similar effect on the ultradian rhythms as a change in work shift has on circadian rhythms.


    As previously discussed the circadian clock is responsible for controlling sleep patterns. Melatonin secretion in the brain actually induces sleep. Commonly depressed patients experience a wide variety of sleep disorders. There is no surprise, that there ought to be connection between disruptions of the circadian cycle and depressive disorders. Generally a decreased amount of deep sleep per night comes just before the onset of depression. Therefore a drastic change in sleep schedule caused by multiple shift changes may result in a disruption of circadian rhythm function. In order to help prevent disruptions in the circadian sleep cycle it is important to maintain a regular sleep schedule, which includes retiring and waking at approximately the same time each day, and sleeping a consistent number of hours each night. This is especially important for people with morningness tendencies because their circadian cycles are less adaptable to changes in behavior. Depressive episodes can be brought on also by life changes that cause stress, such as moving or changing jobs. A regular pattern in one's life can help prevent depression by avoiding situations that cause stress. Many biological rhythms, such as sleeping and eating patterns, could be viewed in the same way. Achieving physiological homeostasis is one of the most basic functions of the human body to maintain internal stability agaist environmental changes. 8. PERMANENT NIGHT-SHIFT WORK CONTRA ROTATING-SHIFT WORK

    In attempts to reduce the problems of night-shift workers, companies have focused on the design of shift systems that minimize the circadian rhythm disruptions. The debates was going on whether or not a permanent night-shift work is a better solution then a rotating-shift work. While it seemed preferable to use a permanent schedule to maximize worker adaptation and decrease disruption of lifestyle, rotating shift systems offered the benefits of minimized adaptation, and thus less disruption, as the number of consecutive night-shifts was reduced. Although, circadian rhythms in permanent night-shift workers never fully adjust to a nocturnal lifestyle, various studies have shown that permanent night-shift workers report fewer sleep or social-related problems than rotating-shift workers do. In addition, permanent night-shift workers reported fewer health complaints than rotating-shift workers did, despite their older mean average age. One explanation of these differences might be that night-shift work is more appealing to some individuals, as it allows flexibility in child-care, and an increased pay scale. As a result, night-shift workers are more likely to organize their lives to take account of unusual work schedules. Rotating-shift workers have not specifically chosen to work at night, but work nights as a part of a rotating schedule. Thus, these workers are more likely to find night shifts a disruption to their typical diurnal schedule.

    An alternative explanation might involve differences in circadian type that allows people to readily adjust to shift work, and those who do not. Researchers have defined circadian types as having three dimensions, including flexibility of sleep timing, ability to overcome drowsiness, and a preference for evening activity as opposed to morning activity. In addition to finding that permanent night-shift workers report fewer health, sleep, social, and domestic complaints that rotating-shift workers, it was discovered that these problems were drastically reduced in workers who had specifically made the decision to work at night. In contrast, it did not appear beneficial to have chosen to work a rotating-shift schedule. The strength of these findings is increased when one considers the large age discrepancy between the two groups: permanent night-shift workers were significantly older than the rotating-shift workers were. Thus, given the association between increased age and intolerance to shift work, one might have expected more problems to be reported by the permanent night-shift workers.



    By establishing an understanding of the environmental factors that influence biological rhythms it is possible to study connections between the resulting shifts and mood disorder or any other mental or physical health problems that may develop. It is important to note also that a significant shift must occur before disorder, will develop. Most living things experience fluctuations in waking/sleeping cycles and feeding cycles for example, however the amplitude of those fluctuations is generally small enough that normal rhythmic cycles can adapt without a harmful impact on health. The important question that arises is when do health problems, develop as a result of shifts in biological rhythms, and what combination of environmental factors leads to those shifts? There are four types of biological rhythms that regulate cycles within the body. The primary type is circadian rhythms that controls performance, endocrine rhythms, behavior and sleep timing, and is regulated by the circadian master clock.

    Although all of these biological rhythms are controlled internally there are a number of external factors that are capable of influencing their regularity. Some of the most prominent examples are exposure to light, and alterations in work shift which change sleeping schedules.. Due to the fact that circadian rhythms can only shift one to two hours each day drastic changes in sleep patterns can have a drastic effect on the circadian clock [8]. There is still a great deal that is not known about the relationship between biological rhythms and mental and physical health disorders.

    Biological rhythms affect many aspects of general health and mental well-being. These rhythms, however, cannot cause a major disorder. This suggests that biological rhythms, when changed drastically, can affect one's well-being for a period of time only, but the effect is not permanent.

    We are also planning to investigate the possibility to phase shift circadian rhythms by means of appropriately timed bright light when subject's sleep schedule is shifted several hours, as in shift work. Phase shifts can be produced in laboratory studies, when subjects are kept indoors. Bright light field studies, in which subjects live at home, show that the use of artificial nocturnal bright light combined with enforced daytime dark (sleep) periods can phase shift circadian rhythms despite exposure to the conflicting 24-h zeitgebers.

     So far, the only studies on the use of bright light for real shift workers have been conducted at National Aeronautics and Space Administration (NASA). In general, the bright light studies support the idea that the control of light and dark can be used to overcome many of the problems of shift work [3]. However, despite ongoing practical applications (such as at NASA), much basic research is still needed.

    The field of physiological rhythms is a relatively new field and there is still not an extensive amount of research papers regarding the importance of biological rhythms. One of the question is whether changes in sleep patterns are a cause of mood disorders, or whether the changes in sleep patterns are result of some other factors such as anxiety or mood disorder.



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     Journal of Occupational Psychology, 64, (1991). 207-218.

    [3] Burgess H.J., Sharkey K.M., & Eastman C.I.: Bright light, dark and melatonin can

     promote circadian adaptation in night shift workers, Sleep Med. Rev.6 (2002).407-420

    [4] Duffy J. F., Rimmer D. W., & Czeisler C. A.: Association of intrinsic circadian period

     with morningness-eveningness, usual wake time, and circadian phase, Behavioral

     Neuroscience, 115, (2001). 895-899.

    [6] Eastman C.I., & Martin S.K.: How to use light and dark to produce circadian adaptation

     to night shift work, Ann Med. 31(1999).87-98.

    [7] Monk T. H.: Parameters of the circadian temperature rhythm using sparse and irregular

     sampling. Psychophysiology, 24, (1987). 236-242.

[8] Santhi N., Duffy J.F., Horowitz T.S., & Czeisler C.A.: Scheduling of sleep/darkness

     affects the circadian phase of night shift workers, Neurosci. Lett. 390 (2005). 187.


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