The researchers, who included James McClain, PhD, MPH, a cancer prevention fellow at the National Cancer Institute, reviewed data on nearly 6,000 Maryland women.
As expected, physical activity was linked to lower cancer rates. But sleep also mattered.
Among the most physically active women younger than 65 -- women who reported getting about an hour a day of moderate physical activity -- cancer was 47% rarer for those who got at least seven hours of nightly sleep. Those findings held regardless of other cancer risk factors.
"What that suggests to us is among younger and middle-aged women, both physical activity and sleep habits might play an important role in reducing cancer risk," McClain tells WebMD.
For perspective, McClain notes that "overall cancer risk among younger women is relatively modest. It's not a highly likely event by any means."
McClain also points out that no one in the study got cancer prevention benefits just by sleeping. "It's not as though you got the benefit, the reduction in cancer risk, if you weren't active," McClain says.
In short, be active and get enough sleep, too.
McClain says he wants to check the findings in larger groups of people. "This is an initial look at a very complex question," he says.
Meanwhile, he suggests that overall lifestyle, not just one or two habits, might be more important for reducing cancer risk.
"You might want to think about your lifestyle as a whole," McClain says. "Think about multiple lifestyle behaviors [such as sleep and physical activity] in more of a holistic perspective."