Feeling jet-lagged? Having trouble sleeping? Is your body clock on the wrong setting? The answer could be to shine a bright light behind your knees, according to two American scientists.

The surprising - not to mention bizarre - finding, reported today, shows that it is not only light falling on our eyes which determine our "circadian rhythms" - the body's internal clock.

Without the stimulus of sunlight, our bodies tend to run on a 25-hour cycle. Scientists had thought that the arrival of light on the retina in a 24-hour cycle helps us to reset that clock to match the natural environment.

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But Scott Campbell at Cornell University Medical College, New York, has shown that shining a bright light on the area behind the knees, known as the popliteal region, has the same effect.

Many animals have multiple photoreceptors to respond to light in this way. In mammals, however, it had been assumed that non-visual body clock light sensors reside in the eyes. But by some quirk of evolution, humans have a circadian rhythm photoreceptor on the back of their knees. For three hours, pulses of light were shone onto the popliteal area of 15 people, though the subjects could not tell if it was on or off because it was hidden.

Shining the light before a certain point in the circadian cycle delayed it; light stimulus after this point produced a phase shift advance. The largest shifts, both advances and delays, occurred at times during which people are normally asleep.

Writing in the journal Science the team comments that other scientists have suggested that when light falls on blood vessels, it increases the concentration of nitric oxide (NO) in the blood. Previous research has already shown that NO can shift the circadian clock. And doctors already know that the back of the knee is one point where both veins and the main artery branch are close to the surface.