Consequences of Disrupting Biological Rhythms

Shift work and jet lag, AO1 and AO2

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AO1 - Shift Work 1

The world requires that some workers have to be awake during the night. However, this disrupts their biological rhythms, with adverse effects.

Decreased alertness

  • Night workers often experience a circadian 'trough' of decreased alertness during their shifts
  • This occurs between midnight, when cortisol levels are lowest, and 4:00am, when core body temperature is lowest

Sleep deprivation

  • Workers who have to sleep by day often experience sleep problems
  • When they finish work it is daytime, and there are other interruptions (e.g. noises outside) and daylight reduces sleep quality
  • Tilley and Wilkinson: daytime sleep is typically between one and two hours shorter than a nocturnal sleep period; REM in particular is affected
  • Poor quality daytime sleep then makes it even more difficult for shift workers to stay awake through the night, especially when they hit the circadian trough
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AO1 - Shift Work 2

Effects on health

  • There is a significant relationship between shift work and organ disease
  • Knutsson et al: individuals who worked shifts for more than 15 years were three times more likely to develop heart disease
  • Martino et al: linked shift work to a range of organ diseases, including kidney disease
  • This may be due to the direct effects of desynchronisation or indirect effects such as sleep disruption
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AO1 - Jet Lag 1

The term 'jet lag' is generally used to refer to the physiological effects of disrupted circadian rhythms, even those that do not derive from jet travel.

Our biological rhythms are not equipped to cope with sudden and large changes - it is estimated that the dorsal SCN takes several cycles to fully resynchronise to abrupt large changes in environmental time.

This is the process we experience as jet lag.

Winter er al: calculated that this is equivalent to one day to adjust to each hour of time change.

Symptoms of jet lag include loss of appetite, nausea, fatigue, disorientation, insomnia and mild depression.

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AO1 - Jet Lag 2

Performance decrement

  • A study of American major league baseball teams that have to travel from coast to coast to play league games
  • The west coast of America is three hours behind the east coast so, when east coast teams have to play on the west coast...
  • ...they experience their clocks going backwards: a phase delay
  • West coast teams experience phase advance when they play teams on the east coast: their clocks go forwards
  • Recht et al: analysed US baseball results over a three-year period
  • Teams that travelled east-west won on average 44% of their games, but the west-east teams only won around 37%
  • This can be attributed to jet lag
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AO2 - Other Factors (Shift Work)

  • The negative effects of shift work may not just be due to the disruption of rhythms
  • They may be due to the lack of sleep associated with having to go to bed at unusual times
  • Shift workers also experience social disruption as well as rhythm disruption
  • It is difficult to meet friends and spend time with family when working nights
  • Solomon: divorce rates among all-night shift workers may be as high as 60%
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AO2 - Other Factors (Jet Lag)

  • Jet lag may also be affected by factors other than the disruption of biological rhythms
  • An individual may sleep badly the night before travelling because of worry
  • Travel itself is tiring, and many holidays involve long hours getting to the airport before flying
  • Drinking alcohol or coffee, constant noise, low-oxygen cabin air and annoying passengers can be cumulative factors
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AO2 - Reducing the Harmful Effects (Shift Work)

Rotating shifts

  • Gold et al: more problems occur when people have to do rotating shifts, where shifts alternate every few days
  • Non-fluctuating shifts, e.g. when an individual works only nights, are less disruptive because the individual can get used to one sleep-wake pattern
  • However, days off are likely to mean temporary changes in sleeping patterns which will disrupt the biological rhythms again

Forward-rotating shifts

  • Bambra et al: these follow the logical order of the day - phase delay - and may be easier on the body and less damaging to health
  • An example of such a shift pattern would be a shift in the morning for one week, and then an afternoon shift the next, and finally a night shift
  • Bambra: also concluded that rotating workers through shift changes more quickly (such as every 3-4 days, as opposed to 7) is better for health and work-life balance
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AO2 - Reducing the Harmful Effects (Jet Lag)

Artificial lighting

  • Moderately effective in resetting rhythms
  • Dim lighting has the opposite effect - it didn't reset the rhythm
  • Gronfier et al: entrained circadian rhythms to longer than 24 hours just by using bright light pulses (modulated light exposure)


  • Natural hormone that induces sleep
  • Has been put forward as a 'miracle cure' for jet lag
  • Herxheimer and Petrie: reviewed 10 studies and found that when melatonin was taken near to bedtime, it was remarkably effective
  • HOWEVER if it was taken at the wrong time of day it actually delayed adaptation

Social Customs

  • Can help to entrain biological rhythms
  • When travelling, it helps to eat and sleep when the clock says
  • Fuller et al: a period of fasting followed by eating on the new time schedule should help entrain biological rhythms
  • This is possibly because some of our body clocks are reset by food intake
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