A new study suggests bright light shining on the back of the knees can help set the body's 24-hour internal clock, 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. The study demonstrates that circadian rhythms in humans can be altered simply by shining light on the backs of people's knees. 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. Researchers hope the discovery leads to new ways to treat patients with sleep and circadian rhythm disorders, such as seasonal affective disorder and jet lag.

Science 1998;279:333-334,396-398