NEW YORK (Reuters) -- Bright light shining on the back of the knees can help set the body's 24-hour internal clock, a new study suggests.
The findings demonstrate there is literally more than meets the eye when it comes to using bright light for changing the circadian clock that tells our bodies when to sleep and when to eat.
They also raise the possibility of new ways for using bright light to treat sleep problems, including those associated with jet lag and seasonal affective disorder (SAD) -- treatments that can be used when the person is asleep.
The circadian clock times a wide range of behavioral and bodily functions by controlling temperature and the release of hormones. And until now it has been widely believed that it is set by daily and seasonal changes in the light that enters through the eyes.
"But our results challenge this belief. The study demonstrates that circadian rhythms in humans can be altered simply by shining light on the backs of people's knees," says Dr. Scott Campbell, director of the Laboratory of Human Chronobiology at Cornell University Medical College in White Plains, New York.
Writing in the journal Science, Campbell and colleague Dr. Patricia Murphy describe an experiment in which volunteers agreed to spend four days at their laboratory in a dimly lit suite. Periodically, they sat in a reclining chair while the backs of their knees were exposed to bright light delivered through a fiber-optic pad in a housing strapped to their legs. The pad was adapted from those used to treat infants born with neonatal jaundice. None of the volunteers knew exactly when the light source was switched on.
The knee was chosen as the site of the experiment because it was far from the eye, therefore minimizing any risk that light shown would enter the subject's eye.
The researchers noted that the light exposure on the back of the knee was associated with shifts in the timing of body temperature changes and in the release of the hormone melatonin from the pineal gland, located deep in the brain.
Moreover, as would be expected with exposure to bright light through the eyes, these shifts depended on the timing of the exposure. For example, light directed to the backs of the knees prior to dawn advanced the circadian clock, so that by the next day the timing of temperature changes and melatonin release occurred earlier.
Campbell says he and his colleagues are now conducting experiments to see if bright light applied to the backs of the knees during sleep can help people with circadian rhythm disorders, such as SAD. SOURCE: Science (1998;279:396-398)