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Bright light behind the knees is just bright light behind the knees


One of the remarkable scientific discoveries of 1998 was the finding that the back of the knees might somehow regulate the body's biological clock. Researchers speculated that travelers and others could reset their internal clocks simply by exposing the back of their knees to bright light. A new study, which repeats the initial experiments under carefully controlled conditions, now seems to debunk the original findings.

The 1998 study, published in Science, seemed to demonstrate, astonishingly, that light absorbed through the skin could adjust sleep-wake cycles in the brain. The idea was that 'photic' signals are carried from the back of the knee to the human brain via the circulatory system. Some invertebrates seem to have a circadian response to light shined on various body parts, but for humans, many researchers have long thought that only the eyes could do the trick.

A flurry of hype followed the original report—Science named it among the year's top studies, and two patented treatments for sleep disorders soon followed. Nonetheless, some scientists challenged the findings at the time because a response to light involving the circulatory system had never been demonstrated in any organism.

The new research, also published in Science, seems to refute the 1998 work effectively by maintaining many of the first study's experimental conditions.

In both case studies, subjects maintained a nighttime sleep schedule during which they were awakened and exposed, for three hours, to one of three conditions: no light, light on the eyes only, or light behind the knees only. For ten days, the same device and the same light intensity were used in both studies. To monitor whether the subjects' circadian clocks were reset, the researchers measured the subjects' production of melatonin, a chemical that regulates sleepiness.

A few critical differences between the old study and the new one, conducted by Kenneth P. Wright Jr. and Charles A. Czeisler of Harvard Medical School in Boston, had a substantial effect on the results. Most important, the Harvard researchers took greater pains to ensure that the subjects' eyes were shielded when light was being shined on their knees. They also kept their subjects in considerably dimmer light during the other 21 hours of each day. Finally, they did not allow subjects to sleep for 12 hours a day, as in the 1998 study.

Because of these changes, the results for the 'light-behind-the-knee' group and the 'no-light-at-all' group were virtually identical. The bodies of the subjects whose eyes were exposed to light seemed to reset themselves, registering the three nighttime hours as the beginning of the day. No such internal-clock resetting occurred in the other groups, regardless of whether or not bright light hit the back of their knees.

See related GNN story
»Beyond Insomnia: Strategies of Circadian Genomics

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Wright, K.P. Jr. & C.A. Czeisler. Absence of circadian phase resetting in response to bright light behind the knees. Science 297, 571 (July 26, 2002).
Campbell, S.S. & P.J. Murphy. Extraocular circadian phototransduction in humans. Science 279, 396-399 (January 16, 1998).

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