Timely Surprises: Biological Clocks Sense Light in Obscure Ways
Travis, John, Science News
When a New Yorker jets off to Los Angeles or Tokyo, it takes just a few seconds to adjust a wristwatch to local time. But as sufferers of jet lag know all too well, the body's internal clock isn't nearly so quick to adapt. It usually takes a few days for a person's biological clock to synchronize with the dusk and dawn of a new time zone.
The lethargy of jet lag sets in during this adjustment period because this internal clock has a profound influence on the human body. It governs when we feel we should sleep or be active. It establishes daily, or circadian, rhythms in characteristics as diverse as hormonal concentrations and body temperature.
Fortunately for travelers, the human biological clock uses environmental cues, primarily light, to continually match its time to that of the outside world. Yet mysteries remain about this entertainment, and two new studies have added to them.
In one report, two researchers counter conventional wisdom with evidence that light shone on skin, rather than the eyes, can reset the biological clock. In the second, scientists suggest that the clock's response to light depends not upon the well-known light-sensing molecules of the eyes but rather on proteins related to plant photoreceptors.
Neither study is definitive, and skepticism remains about both conclusions. Still, scientists who study biological clocks and the circadian rhythms they generate have of late encountered a fair share of surprises. Last year, for example, studies suggested that fruit flies may have independent biological clocks throughout their tissues rather than a single clock in their brains (SN: 12/06/97, p. 365).
"We keep finding, almost every week, that there are remarkable things we don't know about our circadian system. The lesson is we should keep an open mind," says Thomas Wehr of the National Institute of Mental Health in Bethesda, Md.
Wehr's open mind has been tested of late by what may be the most bizarre result ever described in biological clock research. In the Jan. 16 SCIENCE, Scott S. Campbell and Patricia J. Murphy of Cornell University Medical College in White Plains, N.Y., report that light focused on the backs of people's knees reset their biological clock as readily as light shone on the eyes.
If true, this finding might make light therapy, now used by shift workers and people with jet lag, insomnia, or other sleep disorders, much more practical. "You may be able to do it while you're asleep," says Campbell. "Compliance is a major issue [with light therapy]. People don't have time to sit in front of a light box for a couple of hours."
Campbell and Murphy's unusual experiment is in fact the offshoot of a study conducted by Wehr more than a decade ago. Wehr and his colleagues had tested whether light therapy could offer any relief to people with seasonal affective disorder, a depression stemming from the shorter days--and fewer hours of sunlight--of winter.
Exposing their eyes to intense light proved helpful to most of the subjects, but even when their eyes were shielded and the light shone just on the skin, 2 of the 10 people still reported feeling better. Campbell recalls that when Wehr discussed the experiment at meetings, he frequently suggested that someone should repeat the study. "We finally got around to doing it," laughs Campbell.
Instead of relying on a subjective assessment of whether a person feels better, the investigators examined two well-studied physiological measures of the human biological clock--body temperature and saliva concentrations of the hormone melatonin. Both body temperature and melatonin concentrations rise and fall with a circadian pattern, and light provided to a person's eyes can advance or delay those cycles.
To test whether light directed at other body locations can produce similar changes, Campbell and Murphy borrowed a device normally used to treat newborns with jaundice. …