Shining a light on the back of someone’s knee can adjust their “internal clock,” according to a group of scientists. If the findings hold true, they may lead to better treatments for malfunctions of the body’s internal clock, like jet lag, winter depression and insomnia.
The internal clock is centered in a part of the brain called the suprachiasmatic nucleus. Among other things, it regulates the body’s daily cycles of sleep and waking. Throughout the year the clock adjusts a person’s sleeping schedule to the changing lengths of night and day. Scientists had assumed that the clock makes these adjustments based on sunlight that enters the eyes. But the new findings suggest that light can influence the body’s inner workings through at least one other channel—namely, the skin.
The study was headed by Scott S. Campbell and Patricia J. Murphy of Cornell University. The results of the study came as a surprise to scientists, who did not think that the skin had light receptors. Accordingly, many were withholding judgment until the results could be verified. Campbell and Murphy reported their findings in the January 16, 1998, issue of Science.
Besides sleeping and waking, many other processes are regulated by the body’s clock. Resistance to alcohol, susceptibility to pain, and levels of testosterone and other hormones vary predictably according to the time of day. In order to test light’s ability to alter the body’s clock via the skin, the researchers studied two prominent features controlled by the clock: body temperature and levels of the hormone melatonin.
Melatonin, which makes people feel sleepy, starts to increase in the blood around 10 p.m. It starts to drop around dawn. Body temperature keeps climbing during the day, starts to decline around 8 p.m., and reaches its lowest point at about five in the morning.
The researchers measured body temperature and melatonin levels in 15 people whose skin had been exposed to varying amounts of light. The people spent four days in dimly lit rooms. Some of them received doses of light at three-hour intervals between midnight and noon. The light was applied via a fiber-optic pad attached to the back of the subjects’ knees. The other subjects received no light. On the fourth day, researchers found that the timing of the minimum body temperature for those who had received the light treatments had shifted by up to three hours. The shift in the timing of the minimum body temperature suggested that the timing of the subjects’ internal clocks had shifted as well.
“This is the first demonstration that you can affect the human clock without going through the eyes,” Campbell told the New York Times. Scientists speculated that other parts of the body besides the knees may be receptive as well.
If the findings hold true they may lead to more efficient ways to treat disorders of the body’s internal clock, like seasonal affective disorder (SAD), which causes wintertime depression. Scientists believe SAD results from changes in the clock brought on by winter’s short days. SAD sufferers have benefited from sitting under bright lights in the early morning. However, if the body’s internal clock can be corrected by light hitting the skin, patients can undergo the early morning light treatments while still sleeping.
Jet lag, a condition characterized by weakness, fatigue and an inability to concentrate, occurs when people travel across time zones. This causes the rhythms of their internal clocks to go awry. Airplane passengers may be able to correct the problem by attaching light-emitting pads to the backs of their knees.
Scientists are not sure how light hitting the back of the knee registers with the clock in the brain. Some suggested that the light triggers a signal that gets carried to the suprachiasmatic nucleus by the blood pigment hemoglobin. Hemoglobin, like the plant pigment chlorophyll, is sensitive to light.