Disruption of biological rhythms

Black: AO1 - Description
Blue: AO2 - Evaluation - studies
Red: AO2 - Evaluation - evaluative points/IDAs

Shift work

Normally, exogenous zeitgebers change gradually, such as the changing light levels around the year. However, with shift work and jet lag, this change is rapid, and exogenous zeitgebers become desynchronised with endogenous pacemakers. For animals, this could lead to dangerous situations such as an animal leaving their dens at night when dangerous predators are around. In humans, the lack of synchrony may lead to health problems such as gastrointestinal disorders.

Shift workers are required to be alert at night and must sleep in the day, contrary to our natural diurnal lifestyle, and out of synchronisation with available cues from zeitgebers. Night workers experience a "circadian trough" - a period of decreased alertness and body temperature between 12 a.m. and 4 a.m. during their shifts, triggered by a decrease in the stress hormone cortisol. They may also experience sleep deprivation due to being unable to sleep during the day, as daytime sleep is shorter than natural night-time sleep, and more likely to be interrupted.

Czeisler (1982) studied workers at a Utah chemical plant as they adjusted from the traditional backwards shift rotation to a forwards shift rotation. Workers reported feeling less stressed, with fewer health problems and sleeping difficulties, along with higher productivity.  This was due to the workers undergoing "phase delay", where sleep was delayed to adjust to new EZs, rather than the traditional "phase advance", where sleep time was advanced by sleeping earlier than usual. These results suggest that phase delay is healthier than phase advance, as it is significantly easier to adjust to so carries less risk of circadian rhythm disruption.

Czeisler's findings have valuable real-world applications. For businesses employing shift workers, using a forwards rather than backward shift rotation will increase productivity and reduce the risk of employees making mistakes, as well as improve health due to phase delay being easier for the body's circadian clock to adjust to than phase advance.

Gordon et al (1986) found similar results to Czeisler that support the superiority of forward rotation over backwards rotation. Moving police officers from a backwards to a forwards rotation led to a 30% reduction in sleeping on the job, and a 40% reduction in accidents. Officers reported better sleep and less stress.

Studies suggest that there is a significant relationship between chronic circadian disruption resulting from shift work, and organ disease. Knuttson (1996) found that individuals who worked shifts for more than 15 years were 3 times more likely to develop heart disease than non-shift workers. Martino et al (2008) found a link between shift work and kidney disease, and suggested that kidney disease is a potential hazard for long-term shift workers. However, the use of correlations in these studies means that a direct cause and effect cannot be established, and there is not enough evidence to conclude that organ disease is a direct result of shift work - third, intervening variables cannot be ruled out.

The Chernobyl nuclear power plant and the Challenger space shuttle disasters both occurred during night shifts, when performance of workers was most impaired by the circadian trough. The catastrophic nature of these events emphasises the importance that should be placed on healthy shift rotations and the minimising of circadian disruption for workers in order to avoid further disasters.

There are four suggested approaches with which to deal with shift work and its circadian disruption.

• Permanent non-rotating shift work allows the body clock to synchronise with the new exogenous zeitgebers and adapt to a specific rhythm. However, this is unpopular because not many people want permanent night work.
• Planned napping during shifts has been shown to reduce tiredness and improve employee performance - but this is unpopular with both employees and employers.
• Improved daysleep for night shift workers - keeping bedrooms quiet and dark, avoiding bright light and stimulants such as caffeine. However, this method can be disruptive of family life and lead to its own pressures.
• Rapid rotation: rotating shift work patterns every two or three days avoids even trying to adjust to new exogenous zeitgebers. However, it also means that most of the time, rhythms are out of synchronisation, and there is controversy over the suggested effectiveness of this tactic.

The majority of shift work studies (Czeisler, Gordon et al, etc.) involve only male participants, so research into this topic is often gender biased, with the results often unrepresentative of females. Sex differences could mean differences in the levels of neurotransmitters such as orexin and serotonin that affect sleep cycles, so circadian disruption may affect males and females differently. This means that it would be beta gender bias to generalise results from men to women without taking these neurochemical differences into account.

Jet Lag

Jet lag is the disruption in circadian rhythms caused by travelling through multiple time zones very quickly by aeroplane, causing endogenous pacemakers to become desynchronised with local exogenous zeitgebers. This can result in a number of problems including fatigue, insomnia, anxiety, immune weakness and gastrointestinal disruption.

Flying west to east causes worse symptoms and a greater degree of circadian disruption than flying east to west, because phase advance is required in order to adjust to EZ changes when flying east, whereas phase delay is required in order to adjust to EZ changes when flying west. Studies into shift work demonstrate that phase delay is easier for the body's circadian clock than phase advance, causing a lesser degree of disruption and impairment.

Three ways of coping with jet lag have been suggested. Melatonin supplements are widely prescribed in the US to restore melatonin levels when jet lag has greatly disrupted circadian rhythms in order to restore the synchronicity between the internal clock (EPs) and EZs. Planning sleep patterns beforehand has been shown to help adjustment - if arriving in the daytime, stay awake on the plane, if arriving at nighttime, sleep on the plane. Splitting the travel into two days can also help, as each disruption is less severe and people have to make a less significant adjustment on the day of arrival.

Cho (2001) found that airline staff who regularly travelled across 7 time zones had a reduction in temporal lobe size and memory function, providing supporting evidence for the idea that chronic disruption of circadian rhythms due to jet-lag has long-term symptoms of cognitive impairment and neurological damage.

Recht, Lew and Schwartz (1995) provide supporting evidence for the idea that circadian disruption from jet lag is less severe when travelling from east to west, rather than west to east. They studied US baseball teams travelling between time zones for 3 years, and found that teams travelling east to west won 44% of games, whereas teams travelling west to east won only 37%. Although it could be that some teams were simply better than others, the length and sample size of the study means that this should even out. This suggests that phase delay (westward-bound) is easier for the circadian clock to adjust to than phase advance (eastward-bound.)

A significant issue with this study is gender bias, considering that it only studied male participants. Research has suggested that hormonal and neurological differences between males and females can influence sleep behaviour and, by extension, circadian rhythms, so results from this study may not apply to females too - they could be differently affected by circadian disruption resulting from jet lag. It is beta gender bias to generalise these results to both genders without taking potential hormonal differences between genders into account.  

Coren (1996) suggested several real-world applications of research into circadian disruption that can help reduce the severity of jet lag's circadian disruption. Firstly, sleep well before travelling - this will help avoid sleep deprivation. Secondly, avoid stimulants and depressants such as alcohol and caffeine - they will make the symptoms worse by further disrupting endogenous pacemakers. Thirdly, immediately adjust to local exogenous zeitgebers upon arrival - going out into the morning daylight as soon as possible helps resynchronise due to light's function as an EZ. Finally, adjust flight behaviour in anticipation; sleep if you're arriving at night, stay awake if you're arriving in the day.