Would it kill you to go to bed at a reasonable hour? Actually, the opposite may be true. Now researchers are linking late nights to weight gain, addiction—even cancer.
Tamika Handy could star in her own workout video. The 37-year-old mother of five is a nutrition consultant and a fitness coach—with the chiseled biceps to prove it. But for most of her adult life, she had one serious health risk factor: She didn't go to sleep early enough.

After the rest of her family had turned in and all the household chores were done, Handy would log on to the computer to catch up on e-mails and do research. "Right around midnight, I'd think, I need to go to bed," she says, "but by then I couldn't wind down." Her body clock had lost its rhythm.

For most of human history, working late simply wasn't an option. Fires, candles, and lanterns yielded only a fraction of the light that we get today just from opening the refrigerator door. And during the millennia when sunset meant an end to most activity, we evolved to function on a daily physiological cycle known as a circadian rhythm (from the Latin words circa and dies, meaning "around a day"). At least 10 percent of our genes—genes that assist our most basic bodily operations—are cued to operate by the shifts between light and dark that occur during a 24-hour period. Body temperature dips in the morning and peaks in late afternoon. During the day, the stomach emits hunger signals. After nightfall, heart rate and breathing slow. So much of our biology is ruled by this rhythm that some medicines actually work better at certain times of day.

But human beings now stay awake long past the time our genes are programmed to power down. "Bright light beyond sunset is a very unnatural thing," says Steven Lockley, PhD, a neuroscientist at Brigham and Women's Hospital. Lockley and other experts—from psychologists to cell biologists—worry that our electrified night lives are contributing to many major afflictions.

Take cancer, for example. Last August the journal Cancer Causes & Control published data showing that the countries generating the most light at night have the highest incidence of breast cancer. And so many studies have pointed to the dangers of nighttime wakefulness that the World Health Organization has classified shift work that disrupts the circadian rhythm as a probable carcinogen. One reason may be that late hours starve the body of melatonin, a hormone the brain produces only in darkness. David Blask, MD, PhD, the head of chrono-neuroendocrine oncology at Tulane University School of Medicine, has shown that melatonin is a powerful cancer fighter; his experiments have found that breast cancer cells actually stop growing when bathed in about the same amount of melatonin that the brain manufactures at night. "The hormone basically puts tumors to sleep," says Blask.


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