There is some evidence of a link between insufficient sleep and the risk of cancer. In particular, people with circadian rhythm disorders—in which the body's biological clock is disrupted because of shift work, for example—may be at increased risk. A study in the International Journal of Cancer found a relationship between women's irregular work schedules and the rate of breast cancer. Researchers compared 1200 women who had developed breast cancer between 2005 and 2008, with 1300 women who did not have a cancer diagnosis. They found that the rate of breast cancer was 30 percent higher for the women who had worked shifts. Women who had at least four years of night shift work, as well as those with fewer than three night shifts per week (keeping them from ever fully adjusting to one schedule) were at highest risk. Shift work has also been shown to increase the incidence of certain cancers—for example prostate cancer—in men.
Researchers suspect that a disruption in the circadian rhythm could pose a risk for developing cancer, since the body's internal clock affects so many biological functions. One theory is that the suppression of melatonin at night (which comes from exposure to bright light) could be partly responsible. Indeed, scientists have seen this link in animal studies; for example, when they manipulate the sleep/wake cycles of rodents for an extended time, cancers grow faster.
The disturbance in circadian rhythm caused by shift work may also increase the prevalence of gastrointestinal problems. Peptic ulcer disease, for example, is more common in shift workers. To learn more about how to cope with circadian rhythm disorders and develop healthier sleep habits with an irregular work schedule, see the National Sleep Foundations resources on shift work .