Shift work has long been a necessary and unpleasant part of modern life, with more than a quarter of the Canadian work force estimated to be on the job during non-traditional hours in the evenings and overnight.
Those working these shifts often experience disrupted sleep patterns, fatigue and other minor ailments. Now, evidence is mounting that so-called graveyard shifts may be aptly named because they're an under-recognized health hazard, increasing a person's risk of developing cancer, heart disease and other life-threatening illnesses.
Although researchers do not know exactly why shift work is dangerous to health, they're focusing on one unusual culprit: exposure to light at night, and its possible disruption of the body's production of melatonin, a key hormone that tells every cell in the body whether it is day or night.
Experts on the effects of shift work gathered in Toronto yesterday, and the possible role of melatonin, often dubbed the vampire hormone because people produce it only in darkness, emerged as a likely suspect in the hunt for explanations on why working at night holds more risks than doing identical work during the day.
There is "increasing evidence that regular and prolonged work at night may result in adverse health effects including breast and other forms of cancer," observed Scott Davis, chairman of the epidemiology department at the University of Washington School of Public Health in Seattle, and one of the experts.
Many of the key U.S. and international researchers on the health impacts of shift work were at the meeting, which was organized by the Occupational Cancer Research Centre, division of Cancer Care Ontario, and Toronto's Institute for Work and Health.
Several occupational groups have a high prevalence of shift work. About half of health-care employees experience shift work, for instance, as do nearly two thirds of police and security guards.
Among the recent findings is that long-term shift work causes increases in breast, colorectal and uterine cancer risk ranging from 35 per cent to 79 per cent in women, according to Eva Schernhammer, an assistant professor of medicine at the Harvard Medical School in Boston, who has helped track the incidence of cancer in a large group of U.S. nurses.
She said 16 studies have now been conducted on breast-cancer risk and shift work, and most of them have been suggestive of some kind of association, particularly among those who've experienced night and evening employment for long periods, such as 20 years or more.
Although there isn't as much research on men, two studies have detected an elevated prostate cancer risk in males who work non-traditional shifts. There is also emerging research linking shift work to elevated risk of cardiovascular disease, Dr. Schernhammer said.
Dr. Davis says researchers have found it "exceedingly difficult" to determine exactly what it is about shift work that makes it a potential health threat. Traditional explanations have focused on how shifts cause sleep disruption and stress.
Melatonin has emerged as a possible factor because exposure to light at night tricks the body into thinking it is daytime, reducing production of the hormone, which has been found to have potent anti-cancer properties.
Melatonin is made in the brain's pineal gland. Using clues from the amount of light entering the eyes, the gland normally begins increasing production of melatonin in the early evening as it gets dark.
Production peaks around 2 a.m. in the darkness of the overnight hours and then falls off around sunrise. In shift workers, the daily pattern is much more muted, and melatonin synthesis can be cut to nearly nothing in very bright office settings.
The main role of melatonin is fine-tuning the body's circadian, or 24-hour, rhythms of drowsiness and internal temperature. It's best known as the anti-jet lag pill used by people who travel across many time zones to get back into sync with the different sleeping hours in their new surroundings.
A U.S. researcher, David Blask of Tulane University School of Medicine in New Orleans, has conducted studies of human breast-cancer tumours implanted into rodents and found their growth rate is regulated in part by melatonin levels.
When melatonin levels drop, "tumour growth rates increase dramatically," Dr. Blask says. "During the night, tumours go to sleep."
In one experiment, Dr. Blask pumped blood from women who had been exposed to strong light at night - and therefore had low amounts of melatonin - into tumours, causing the malignancies to grow rapidly. When melatonin-rich blood was used, tumour growth fell to negligible levels.
There may be another possible health hazard with night shifts that is also connected to light. People who work shifts may not be exposed to enough sunlight during the day.
Dr. Davis said there is speculation that shift workers may be short of vitamin D, the so-called sunshine vitamin, because they typically sleep during the day. Most of the vitamin D that people have in their bodies they make themselves in bare skin exposed to strong summer sunlight. Insufficiencies of vitamin D have also been linked to several cancers and heart disease.