Light Shone on Body Parts Can Reset Rhythms

New research shows that bathing different parts of the body with light, even the knees, can reset your biological clock.

“Until we did this study, it was widely assumed that in mammals, including humans, their biological clocks could not be affected by light except through the eyes. So this is quite a surprising result,” said chronobiologist Scott Campbell of Cornell University.

It is especially surprising because the backside of the knee--called the popliteal region--doesn’t have any light-receiving organs, such as the rods and cones in the eye. So it seemed unlikely that anything but the eye could be used to reset one’s biological clock.

This discovery, Campbell said, “makes us more like plants and other animals,” including fish, lizards and birds, whose circadian rhythms respond to light, even if they’re blind. The only caveat, Campbell said, is that the body part being illuminated has to be well-vascularized, meaning an area where blood flow is ample.

The new results, published Thursday in the journal Science, could be important in treating people who have such problems as seasonal depression and sleep disorders linked to circadian rhythm, Campbell said. “Light exposure to the eyes has been shown to be a pretty good treatment for these.”

Now, he said, the new work “opens up the possibility of resetting the clock during sleep.”

Researchers Dan Oren, at Yale University, and Michael Terman, at Columbia University, suggested that light may be picked up by hemoglobin molecules in red blood cells, which could interact with the blood-borne compound melatonin, to alter the biological clock.