Everyone knows that caffeine can keep you awake, but a new study shows that the world's most popular drug can actually interfere with your body's internal sense of time.

"The circadian clock is way beyond 'sleep and wake,' " says Kenneth Wright, a sleep and circadian physiologist at the University of Colorado, Boulder. "The circadian clock is present in cells throughout our entire body. It's in your fat cells; it's in your muscle cells. It's in your liver, for example, as well as in your brain."

And irregularities in the circadian clock have been linked to everything from obesity to cancer.

Scientists have had hints that caffeine could alter cellular timekeeping, from studies in bread mold, green algae and sea snails. But Wright wanted to know if it did the same thing in people.

"To our surprise, no one had really tested this question," he says.

He and his colleagues got volunteers to do a 49-day sleep study that involved testing saliva for melatonin, a sleep hormone that typically surges at night ...Read more »