Imagine preventing jet lag simply by strapping a light to your leg and leaving it on while you sleep or read in your airline seat. This scenario was suggested by a report published in Science 4 years ago describing a new way to reset the human internal clock. The discovery captured the imagination of scientists and entrepreneurs as well as the public. But researchers now have cast doubt on the finding--and on prospects for commercializing this patented form of light therapy.
In the original study, Scott Campbell and Patricia Murphy of Cornell University Medical College in New York state reported that by shining light on the backs of the knees of human subjects, they could shift the so-called circadian clock that governs sleep-wake cycles (Science, 16 January 1998, p. 396). The backs of knees are laced with blood vessels close to the skin, and the scientists believed it might be possible to transmit a timing signal through blood. Their findings flew in the face of conventional wisdom that light can only reset the biological clock via the eyes, and many researchers in the field were skeptical.
So circadian clock researchers Kenneth Wright and Charles Czeisler of Harvard Medical School in Boston repeated the experiment, with a few improvements. For instance, Campbell and Murphy had used body temperature as the indicator of circadian phase for all their subjects. But a later study reported that body temperature can be off by as much as 5 hours from circadian phase, so Wright and Czeisler tracked levels of the nighttime hormone melatonin, thought to be a more precise measure. The result, published in the 26 July issue of Science, will disappoint jet-setters: Light to the back of the knees did not shift the circadian clock.
Many clock researchers praise the new work: "This paper will have a high impact," says clock researcher Shin Yamazaki of the University of Virginia in Charlottesville. "The experiment ... is really well controlled." Campbell, however, counters that the original study was properly controlled and has been replicated in his lab.