They made the discovery while investigating the internal biological clock that governs our daily sleep-wake cycle.

Scientists have long known that our 24-hour “circadian rhythm” is regulated by a group of cells in the hypothalamus region of the brain. These cells, which represent the body’s main clock, are sensitive to changes in light conditions registered through the optic nerve in the eye.

Researchers at Harvard Medical School and Beth Israel Deaconess Medical Center in Boston have now pinpointed a second clock that is set by the availability of food. Their study, published today in the journal Science, is based on research on mice. But they believe all mammals, including humans, possess an internal food clock, too.

Clifford Saper, the senior author of the study, said this second clock probably takes over when food is scarce. It may have evolved to make sure mammals don’t go to sleep when they should be foraging for food to stay alive.

Dr. Saper says long-distance travellers can probably use this food clock to adjust rapidly to a new time zone.

“A period of fasting with no food at all for about 16 hours is enough to engage this new clock,” he said in a statement released with the study. Once you eat again, your internal clock will be reset as though it is the start of a new day.

Although more research is needed to confirm the findings, travellers could probably activate the second clock in the following way: On an overnight trip to Europe, fast before the flight and don’t eat on the plane. After you arrive the next morning, eat a nutritious meal.

Normally, the body’s light-driven main clock takes a long time to adapt to a foreign location, adjusting by only an hour or two each day.

“The neat thing about this second clock is that it can override the main clock … and you should just flip into that new time zone in one day,” Dr. Saper said in an interview.