Shift work and disrupted circadian rhythms

  1. Interesting info on health risks of shift work (which we all know, right?)

    Shift Work Dangerous to Your Health

    A growing body of evidence suggests the modern world's move away from 9-to-5 jobs is taking a toll on workers' health -- and society's pocketbook.

    Shift work appears to throw off the body's natural rhythm enough to lead to chronic sleep disturbances, gastrointestinal problems and even heart disease. With increasing economic and social
    demands, we are rapidly evolving into a 24-hour society.

    Working on non-traditional schedules, which may include staying up all night, throws off the body's circadian rhythms. These rhythms are governed by the body's internal "clock" and help control the sleep/wake cycle as well as a host of biological processes such as hormone production and blood pressure. And the human clock has evolved to match the light/dark cycle.

    Attempts to sleep at inappropriate phases of the circadian cycle will usually result in shorter sleep episodes and more awakenings. Such attempts are frequent in workers on night shifts.

    The investigators compare the short-term effects of shift work to symptoms of jet lag, such as daytime sleepiness, disturbed sleep, gastrointestinal problems and blunted alertness. The difference, the authors note, is that travelers will eventually adapt to their environment, while shift workers live out of synch
    with their daily surroundings.

    And over time this may take a toll. A number of studies indicate shift workers face a higher risk of heart disease -- possibly due to the metabolic effects of working and sleeping unusual hours.

    There is also a price for society. For one, sleep loss may make shift workers less productive. And accidents that stem from sleepiness, such as car accidents, exact a high cost. According to the researchers, "sleepiness-related accidents" cost the US about $16 billion a year.

    Employers and individuals need to be aware of the major performance and alertness decrements associated with night activity and how to best manage and counteract them.

    Some tactics that may help circadian rhythms adapt to unusual hours include getting an adequate amount of sleep -- whatever time of day that is -- controlling caffeine and alcohol intake, and sleeping in a dark, quiet environment.

    Biological time is not only scientifically important, but it also greatly affects the productivity and health of a nation. The cost to the nation's health of working out of phase with our biological clocks is probably incalculable at present.

    This is such an important topic that I felt a condensed version of the article would be helpful:

    The 24-h society is an environmental challenge that outstrips our biological adaptation to the
    natural 24-h cycle of light and darkness. In the course of evolution, the behavior and physiology of most organisms, including human beings, have developed internal temporal characteristics.

    It is thought that by timing behaviors such as sleep so that they complement the organism's spatial ecological niche, internal stability is maintained and the chances of an organism's survival are increased.

    In addition to health problems there is a substantial cost to the economy in terms of decreased efficiency and productivity. The cost of sleepiness-related accidents can vary substantially, but in general, the estimated total cost of such accidents per year is US$16 billion in the USA, and US$80 billion worldwide.

    Circadian (about 24 h) rhythms, are controlled by a master biological clock. In mammals, the master
    biological clock is located in the suprachiasmatic nuclei of the hypothalamus.

    At the subcellular level of organization, circadian rhythms are generated by transcriptional and translational feedback loops involving multiple clock genes.13 The precise periodicity (or cycle length) of the biological clock is known to be genetically determined,14 and variation in clock genes is thought to be related to individual differences in natural wake and sleep times.

    The biological clock generates and maintains circadian rhythms in most physiological, biochemical, and behavioral variables, for example:
    core body temperature
    blood pressure
    mental performance

    It is also responsible for the synthesis and secretion of many hormones including

    growth hormone


    A reliable and extensively researched marker of biological-clock activity is the rhythm of melatonin. Melatonin is the principal hormone of the pineal gland. It is synthesized and secreted at night in both day-active and night-active species, thereby acting as a signal for the length of day and night.

    In human beings, sleep is normally initiated during the rising phase of the melatonin rhythm and declining phase of the body temperature rhythm.

    Attempts to sleep at inappropriate phases of the circadian cycle, for example during the declining phase of melatonin and rising phase of body temperature, will usually result in shorter sleep episodes and more awakenings. Such attempts are frequent in workers on night shifts.

    Light is the major synchronizing agent for mammalian circadian rhythms. Results of studies have shown that exposure to even low light levels (100 lux), similar to that found in offices and living rooms, will substantially affect the phase of human circadian rhythms. However, without scheduled activities and sleep, such intensities seem incapable of maintaining optimum synchronization to the 24-h day.

    Responses to light depend on the time of exposure in relation to the internal biological clock: exposure to light just after the body temperature minimum will advance the phase of circadian rhythms, whereas exposure before the body temperature minimum will induce delays.

    Core body temperature is usually at a minimum around 4 to 6 AM, but it can be substantially displaced by shiftwork, jet-lag, and other situations.

    In continuous darkness or in dim domestic intensity light and in the absence of other important time cues such as an imposed sleep-work schedule, human rhythms free run, or become desynchronized from the 24-h day and express the underlying periodicity of the biological clock.

    This is often seen in blind people who have no conscious light perception. Rhythms can be synchronized by weak time cues, but have an abnormal phase relation with the environment.

    An example is the tendency to oversleep in winter (dim light), which in polar regions (especially in individuals with no behavioral impositions such as scheduled sleep wakefulness and work times) can become an overt free run.

    For those working indoors during a normal day (08001700 h), bright natural early morning light is only seen in the summer in the higher latitudes of temperate or polar regions, and this early morning light exposure might well result in earlier circadian phase.

    Timed exercise can also shift the human biological clock, however, to date mainly phase delays have been shown. Appropriately timed administration of melatonin can, in addition to inducing sleepiness, phase shift and synchronize the human circadian system.

    In countries where melatonin is freely available, it is extensively, indiscriminately, and no doubt often inappropriately, used as a treatment for circadian rhythm disorders and as a sleeping pill.

    Shiftwork and Jetlag

    A key characteristic of the biological clock is its ability to re-adjust (either by phase advancing or delaying)to changes in the environment. On average, the clock shifts about 1 h per day in the absence of countermeasures.

    Symptoms of jetlag are thought to be caused by desynchronization of circadian rhythms from the external environment, the transient change in the phase relationship of individual rhythms, and perhaps changes in the amplitude of rhythms.

    About Two-Thirds Of Travelers Report Having Jetlag.

    Symptoms of jet-lag include:

    daytime tiredness
    difficulty initiating sleep at night (after eastward flight)
    early awakening (after westward flight)
    disturbed night-time sleep
    impaired daytime alertness and performance
    gastrointestinal problems
    loss of appetite and inappropriate timing of defecation and urination

    Such symptoms can seriously impair a person's performance and ability to function, in part because of the reduction in sleep quality and quantity, and because performance and alertness rhythms will take several days to resynchronize.

    In the long-term (eg, after 4 years), chronic disruption of circadian rhythms from regular transmeridian travel might result in cognitive deficits (decreased short-term memory, slower reaction time) and changed physiological parameters (such as cortisol concentrations).

    Because of their rapidly changing and conflicting light-dark exposure and activity-rest behavior,
    shiftworkers can have symptoms similar to those of jetlag.

    Although travelers normally adapt to the new time zone, shift-workers usually live out of phase with local time cues.

    Shift-work schedules are generally classified in terms of the speed (rapid or slow) and direction (forward or backward) of rotation. The issue of which schedules are preferable from the perspective of sleep and biological rhythm research is contentious.

    On the one hand, in rapidly rotating schedules, which incidentally are rarely used in North America, the biological clock maintains a normal phase and workers are thus able to continue their conventional activities during off-duty days without symptoms of internal desynchrony.

    However, the problem with such schedules is that shifts can, and often do, coincide with the time of day when the biological drive for sleepiness is high and performance is low.

    By contrast, a slow rotation schedule is conducive to circadian adaptation. During days off duty, workers typically revert to the conventional day-active pattern. In Antarctica and in one North Sea oil rig shift schedule complete adaptation is found, but such situations are rare.

    In the offshore situation, many more complications are seen in sleep and performance in the rollover shift than with 2 weeks of night shift.

    The theoretical notion of directional asymmetry in circadian adaptation to rotating shift schedules is based on the same principles as for time zone travel; forward (clockwise) shift rotation would result in more rapid adaptation than backward rotation.

    In addition to disruption of sleep, abrupt changes in time cues might have negative effects on other
    physiological systems. Compared with the effects of sleep, few studies have examined the effects of
    shiftwork on cardiovascular, digestive, immune, and reproductive systems, all of which are rhythmic in nature.

    Epidemiological studies are problematic; we know that people who are intolerant to shiftwork tend to select themselves out of such occupations.

    A review of studies that investigated shift work and risk of cardiovascular disease claimed that on balance, shift-workers have a 40% increase in risk.

    Glucose tolerance is also known to deteriorate in the evening, and there is evidence that increased
    peripheral insulin resistance might contribute to this effect. Resistance to insulin is a putative risk factor for cardiovascular disease and type 2 diabetes mellitus, and again, this could explain the raised incidence of disease among shiftworkers.

    Strategies have been developed to enhance circadian adaptation to shift-work schedules and time zone changes. Factors that promote sleep hygiene are advised, such as:
    adequate sleep
    sleep in a quiet and dark environment
    control of the use of caffeine and alcohol
    timing sleep (with or without the use of hypnotic agents) to the desired sleep time relative to the
    new time zone or shift schedule

    As described earlier, exposure to light can phase shift circadian rhythms. Therefore, scheduled bright light exposure and avoidance of light (possibly by use of dark goggles) might be useful in accelerating adaptation.

    Most field studies and laboratory-simulated phase-shift studies report that correctly timed administration of the hormone melatonin is also able to moderately shorten the time taken for circadian adaptation.

    However, there is little evidence for optimum dose or formulation, and there is no information on long-term safety. Further research is needed to examine how combined administration of bright light and melatonin could be used to develop effective, reliable, and practical treatment strategies.

    It is not always desirable to adapt the circadian system to new shift schedules, for example in rapidly rotating shifts, because sleep and activity on rest days will be compromised. Similarly, when travel to a new time zone is for a short time (eg, 1 or 2 days), circadian re-adaptation might not be worthwhile.

    In such cases, short-term strategies can be used to maintain alertness and performance, especially during early morning hours, and to improve sleep, without shifting the biological clock.

    Sleep Loss and Sleepiness

    Sleep loss is obviously the most important immediate consequence of night work.

    In general, sleep loss will result in performance deficits, including increased variability in performance, slowed physical and mental reaction time, increased errors, decreased vigilance, impaired memory, and reduced motivation and laxity.

    There is no consensus on the extent of impairment resulting from a given amount of sleep loss. Generally, complex performance tasks seem to be more sensitive to the effects of sleep loss than simpler tasks. It is of interest to note that the legal blood alcohol concentration limit for driving in the UK, USA, and Canada is 0.08%, in Australia is 0.05%, and in Sweden is 0.02%.

    The decrements in performance recorded after extended wakefulness have important implications for shiftwork, since a substantial number of shiftworkers are reported to be awake for at least 24 h on the first night shift in a roster.

    In reality, the temporal pattern of alertness and performance is thought to be the result of an interaction between circadian and homoeostatic influences. The homoeostatic aspect, also referred to as sleep debt or sleep pressure, will increase as a function of the duration of wakefulness and dissipate during a subsequent sleep episode.

    Models have been developed to predict alertness levels as a function of these two factors. Such findings can be usefully applied to shiftworkers to determine optimum sleep-wake schedules which keep alertness and performance at a maximum during the shift.

    An important issue associated with napping is sleep inertia, which is the feeling of disorientation and performance impairment that happens after awakening. Estimates of the duration of sleep inertia vary substantially, ranging from 1 min to 4 h. Generally, sleep inertia seems to be worse when the individual is awoken during deep, slow-wave sleep, and after previous sleep loss.

    The Lancet September 22, 2001;358:999-1005
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  3. by   fedupnurse
    Thanks RNKitty! I have known for years that this crazy shift I work takes a toll. All th more reason we should get much bigger bucks to work it. It is nice to see what I and all other night shifters have lived for years be put in writing and validated.
  4. by   a-rose
    Thanks for your information!
  5. by   oramar
    There is an age factor also. From age 19 to 38 I adjusted very well to nightturn, the problems started for me about age 40. Nothing like trying to cope with menopause and sleep deprivation at same time. I can be flipped into a major depression just by going on nightturn. No other stressors are necessary.
  6. by   boobaby42
    I agree. Age 40+ is a turning point.