By Amanda MacMillan
MONDAY, Nov. 17, 2008 (Health.com) — Regular physical activity may lower a woman’s overall risk of cancer, suggests a new government study—but only if her workouts don't cut into a good night’s sleep. Otherwise, lack of shut-eye appears to cancel out much of exercise’s protective benefits.
The link between physical activity and a reduced cancer risk is well established, says James McClain, PhD, a cancer prevention fellow at the National Cancer Institute and lead author of the study. So far, there is little evidence that sleep impacts cancer risk, but not getting enough does appear to negatively affect many of the same hormonal, immune, and metabolic functions in the body that exercise is known to improve.
McClain’s study—presented at the American Association for Cancer Research’s seventh annual conference—followed 5,968 women for almost 10 years, during which 604 of them developed some form of cancer. Based on self-reported data, those in the top half of physical activity levels showed about a 20% reduction in cancer risk compared to those who exercised less frequently.
Among the more active women 65 and younger, however, those who slept for fewer than seven hours a night saw much of that benefit negated. Their cancer risk was greater than those who exercised but slept more—but still lower than those who exercised the least.
“When you look at the previous research on physiological effects associated with increasing activity as opposed to sleeping adequately, you can see they appear to go in different directions,” McClain says. “Though this was a small study, it supports the hypothesis that sleep might modify the relationship between physical activity and cancer.”
Many health problems have been linked to a lack of sleep, says David Rapoport, MD, director of the Sleep Medicine Program at New York University's Langone School of Medicine.
Even short periods of sleep deprivation can promote glucose intolerance, increasing a person’s risk for diabetes. It can also cause imbalances in ghrelin and leptin hormone levels—which regulate appetite—prompting people to overeat and gain weight. Research also suggests that it can increase C-reactive protein (a marker of inflammation and known risk for cardiovascular problems) in the blood, alter coagulation and platelet function, and raise blood pressure.
It’s not clear from this study whether cancer risk is truly affected by lack of sleep, says Dr. Rapoport, but it is certainly a possibility.
“One link might be through the changes in immunity induced by sleep loss; reduced immunity may cause an increase in cancer risk,” he speculates. On the other hand, those who aren’t getting seven hours of sleep may be kept up by another condition—anxiety, sleep apnea, or an undiagnosed illness—that could be the real risk factor.
There was no link between sleep and cancer risk in the other women in the study, particularly in those older than 65.
“We split the data in this way because older adults, especially women, tend to report more insomnia as they age, and sleep patterns in general start to change,” McClain says. While this may be partially due to injury, illness, or other complications of aging, some research suggests that older adults need less sleep in general. Sleep duration did not significantly affect cancer risk for women of any age in the lower physical activity bracket.
McClain speculates that the same study performed on men would have similar results.
Of course, this begs the question: Which is more important—rising an hour early to squeeze in a morning workout, or savoring that extra time in bed?
“The important thing to remember here is that nobody was protected unless they were active,” McClain says. “So I certainly don’t think the take-home message is to sacrifice activity to get more sleep. But I think it needs to be more of a conscious thought that both physical activity and sleep are important factors for young to middle-aged women.”
Dr. Rapoport agrees that finding the right balance is key: “Both sleep and exercise seem to be important, and one certainly feels better if one gets both. So unless you are lucky enough to not need the sleep and feel well without it, I would try to exercise in a way that does not cut into your rest.” Most adults need seven to eight hours of sleep to function optimally the next day.
The good news is that sleep and exercise already have something of a relationship: Getting enough of one can make it easier to accomplish the other.
“Exercise is known to help with sleep if you don’t do it just before bedtime,” possibly because it promotes overall good health and helps enhance the body’s circadian rhythm, which promotes activity during the day and resting at night, says Dr. Rapoport.
While specific exercise advice couldn't be drawn from this study, McClain refers to the government's physical activity guidelines that suggest getting at least five hours of moderate exercise or two-and-a-half hours of intensive exercise each week. He adds, "Getting more than just the minimum recommendation is a good place to start for protection against cancer."