Workplace Health: Is shift working bad for your health? Possibly, writes Dr Muiris Houston, Medical Correspondent
The level at which your body functions oscillates between high and low depending on the time of day. This 24-hour cycle is called a circadian rhythm, which comes from the Latin circa dies meaning about one day.
The body's organs and systems either increase or decrease their level of activity depending on the time of the day. Hormones such as cortisol and even the temperature of our bodies change constantly. Cortisol is linked to alertness and a chart of its levels is indicative of how "switched on" we feel at any given time.
Shift work is generally organised into four types: day work; permanently displaced work hours; rotating shift work; and roster work.
Day work involves working between 7 a.m. and 7 p.m. Permanently displaced hours requires you to work either a morning shift (6 a.m.-2 p.m.), an afternoon shift (2 p.m.-10 p.m.) or a night shift (10 p.m. to 6 a.m.).
Rotating shift work involves alternating between these shift patterns. In a three-shift work system you may be expected to rotate from one cycle to the next.
But how does shift work affect our health? The most obvious and indeed the dominant problem experienced by shift workers is disturbed sleep.
At least three quarters of those working shifts experience difficulty sleeping. Studies of rotating shift workers have shown that daytime sleep is one to four hours shorter than night sleep.
Research has also shown that permanent night workers sleep better than people who rotate through the night shift.
Night shift workers also complain of fatigue and reduced alertness. This sleepiness is worse towards the early morning when cortisol and body temperature levels are at their lowest. Workers can easily fall asleep; it is also a time when workplace accidents are at their highest.
Symptoms of gastrointestinal disturbances are more common among night shift workers. Stomach ulcers occur in up to 15 per cent of employees who work rotating night shifts. Other disorders such as gastritis and heartburn are also more common in this group.
A number of studies have shown a higher incidence of coronary heart disease in male shift workers.
In one piece of research, spanning 15 years, 504 paper mill workers found a direct relationship between years of shift work and incidence of heart disease. Increased risk was found to be independent of other factors such as smoking and family history.
According to a controversial study of breast cancer and night work published in 2001, women who regularly work nights may be up to 70 per cent more likely to develop cancer than their counterparts in day jobs. The researchers speculated the link between shift work and breast cancer may be the hormone, melatonin, which has been shown to inhibit the growth of human breast cancer cells. The hormone is produced mainly at night, regulated by a normal circadian rhythm. So, a person who stays awake all night under artificial lighting will produce less melatonin, which could, in theory at least, increase the risk of breast cancer.