According to a 2004 survey by the U.S. Bureau of Labor Statistics, a little more than 3 percent of full-time wage and salary workers work night shifts - defined as shifts between the hours of 9 p.m. and 8 a.m. Another 2.5 percent work rotating shifts, which can include overnight shifts.
The Cleveland Clinic in Cleveland, Ohio, reports that overnight workers suffer from a condition called Shift Work Sleep Disorder, with symptoms including insomnia, excessive sleepiness, difficulty concentrating and lack of energy. Lately, there have been some studies suggesting that overnight work can have more serious health effects.
Fact Check: Is working overnight hazardous to your health?
– The Fred Hutchinson Cancer Research Center in Seattle, Washington, conducted a 2001 study that compared 800 Seattle-area women who had been diagnosed with breast cancer to 800 women who had not been diagnosed with breast cancer. Researchers concluded that women who had worked the overnight shift had a 60 percent increased risk for breast cancer. In addition, they said, "The risk of breast cancer increased with each additional hour per week of graveyard-shift work." Despite those findings, the author of the report cautioned, "It is too early to say that (working at night) 'causes' breast
– A 2001 study by Boston, Massachusetts' Brigham and Women's Hospital of 78,562 nurses noted "a moderate increase in breast cancer risk among the women who worked 1-14 years or 15-29 years on rotating night shifts. The risk was further increased among women who worked 30 or more years on the night shift."
– In 2008, the Danish government recognized breast cancer after night-shift work as an industrial injury, and granted compensation in 37 cases. According to Denmark's Occupational Diseases Committee, "the injured person typically had night-shift work for at least 20-30 years, and at least once a week."
– In 2009, Brigham and Women's Hospital studied 10 healthy volunteers over 10 days. Researchers say hormone levels "showed the largest deviations" when the volunteers were sleeping during the day and awake all night. They concluded that abnormal hormone levels "may lead to increased health problems common in shift workers." Those problems, the researchers said, include increased risks of obesity, hypertension and diabetes."
While much more research needs to be done into the health effects of night-shift work, studies suggest possible links between night shift work and serious health problems.
– CNN's Tim Langmaid contributed to this report.
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