Human circadian rhythms that govern sleep, body temperature, and other regular cycles apparently can be influenced by shining bright light on the body--even if the eyes cannot see it. The finding, reported in tomorrow's issue of Science, suggests that blood vessel-rich spots on the body, such as behind the knees, might eventually serve as sites for treating sleep disorders.
As the seasons change, the human body adjusts its 24-hour cycles of sleep and waking to the lengths of days. The master timekeeper of this circadian clock is believed to be a bundle of nerves called the suprachiasmic nuclei. Because this bundle sits on top of the brain's optic nerve channel and receives impulses directly from the retinas, researchers have argued that the eyes help set the clock. But recent research suggests that the body may have other tricks for keeping in synch with the seasons: light-sensitive compounds carried by the blood, such as hemoglobin and the liver's bilirubin. These compounds seem to influence production of melatonin, a hormone that helps control sleep cycles as its levels rise and fall through the day.
Chronobiologist Scott Campbell and his colleagues at Cornell University Medical College in White Plains, New York, set out to test whether human circadian rhythms could be influenced by light that doesn't reach the eyes. They chose the region behind the knees, Campbell says, because it has plentiful blood vessels near the skin surface. Fifteen healthy volunteers were enrolled in the experiment and spent stints of 4 days and nights in artificially lit isolation chambers. During the second night of each stint, the researchers shone blue-green light--which quickly influences the sleep cycle--onto the backs of the subjects' knees for 3 hours. The result: Body core temperatures and melatonin outputs of the test subjects--but not controls--shifted consistently in response to the light exposure, in some cases by 3 hours.
This makes sense, Campbell says. He points out that many other vertebrates have light-sensitive systems that don't involve their eyes, adding that more ancient ways of setting the body's clock may still exist: "Just because the eyes are the evolutionary choice in mammals doesn't mean the other systems have disappeared entirely." Extremely vascular areas such as behind the knees might allow doctors to use these vestiges to treat sleep disorders while subjects are sleeping.
The experiment is "an excellent first test," according to Michael Terman, a physiological psychologist at New York State Psychiatric Institute in New York City. But the feasibility of treating sleep disorders with the technique will depend on whether it can produce more than a brief circadian disruption, and it remains to be seen whether the technique can accomplish that, says Terman.