NEW YORK (Reuters Health) - In 1998 it was widely reported that shining bright light on the back of the knees may help re-set the body's internal clock, sparking hope not only for jet-lagged travelers but also for those suffering wintertime blues.

Now, two researchers at Harvard University may have pulled the plug on the much-ballyhooed finding.

The body's circadian clock controls a wide range of functions, such as temperature and the release of hormones. When it gets thrown off--as it's apt to do when traveling between time zones--there is little a person can do except wait for it to reset itself.

This "clock" is light sensitive, receiving its primary signaling from a person's eyes. Previous studies of laboratory animals have shown that, in the absence of eye signals, the circadian clock cannot be altered, even in the presence of light, Dr. Kenneth P. Wright of Harvard Medical School in Boston, Massachusetts, explained in an interview with Reuters Health.

These facts made the previous study's findings on behind-the-knee light all the more surprising, according to the researcher.

So Wright and colleague Charles A. Czeisler conducted a study to see if they could replicate the original study's findings. Their results are published in the July 26th issue of Science.

The researchers evaluated 22 healthy volunteers who lived in a laboratory for 10 days and were exposed to behind-the-knee illumination of the same duration and intensity as in the original study. Wright and Czeisler also measured levels of the sleep-regulating hormone melatonin in participants' blood.

In addition, the researchers took care of some reported shortcomings of the 1998 study--including the fact that participants' eyes were not shielded while the backs of their knees were illuminated.

Wright and Czeisler found that back-of-the-knee light had no effect on their subjects' sleep cycles and melatonin levels.

"Our findings are inconsistent with the original findings," Wright told Reuters Health. "They don't support the idea that shinning light on the back of the knee can reset or shift the body clock."

SOURCE: Science 2002;297:571.

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