David E. Blask, MD, PhD, of Bassett Research Institute in Cooperstown, N.Y., reported the findings at this week's annual meeting of the American Association for Cancer Research.
"This breast cancer rev-up mechanism gets revved down by melatonin," Blask said at a news conference. "Nighttime melatonin is a relevant anticancer signal to human breast cancers. Ninety percent of human breast cancers have specific receptors for this signal."
The hormone seeps from a pea-sized gland in the brain when the lights go out at night. It's the reason you get sleepy when it's dark. Blask and colleagues found that melatonin puts cancer cells to sleep, too.
Blask's team exposed lab mice with human breast cancers to constant light. Tumor growth skyrocketed.
"With constant light, tumors grow seven times faster and soak up incredible amounts of linoleic acid," he says. "During the day, the cancer cells are awake and linoleic acid stimulates their growth. But at night cancer cells go to sleep. When we turn on lights at night for a long time, we suppress melatonin and revert back to the daytime condition."
The finding may explain why nurses who often work the night shift have high rates of breast and colon cancer.
Blask says clinical trials are under way to see whether melatonin supplements can help treat cancer. It may also help in other ways.
"When you take melatonin prior to normal onset of sleep, it will [jump-start the sleep cycle]," he notes. "Many cancer patients suffer from sleep problems. Melatonin may also improve the quality of life in cancer patients by helping them sleep."
Arizona Cancer Center researcher David Alberts, MD, notes that there is a lot of interest in melatonin as a sleep inducer. However, he worries about the safety of over-the-counter melatonin supplements.
"The issue is safe dosing of melatonin," he said at the AACR news conference.
Provided that melatonin supplements actually contain the hormone, Blask isn't worried about overdose.
"In human studies, melatonin has basically no toxicity," he tells WebMD. "Now it takes very little melatonin to stimulate nighttime sleepiness -- on the order of three-tenths of a milligram. But you can't overdose with melatonin. People have taken gram quantities. Its nastiest side effect is sleepiness."