It has been long known that plants respond to light on a 24-hour or circadian rhythm. Plants often open their leaves during daylight hours and close them during periods of darkness while others bloom only in periods of light. Such plant behavior is largely regulated by internal clocks that are reset by light as the days grow longer or shorter.
The exact mechanism by which plant rhythms work is unknown but it is thought that light-sensitive pigments such as chlorophyll and phytochrome are chiefly responsible. Animals also exhibit behaviors driven by the amount of available light ranging from hibernation to bird migration. In the animal kingdom it was assumed such behavior could only be initiated by the photorecptor molecules located in the rods and cones of the eye. Research published in a recent issue of Science shows that, in humans at least, this is not necessarily the case.
Chronobiology, the name given to the science of studying biological rhythms in plants and animals, has been extended to humans in recent years. Research published in a 1996 issue of Nature demonstrated that humans are highly sensitive to even slight differences in amounts of available light. Researchers associated with Harvard Medical School studied the effect of light variation on the body temperatures of a number of male volunteers. Some stayed in a constant light intensity except for darkness during sleep periods while others were subjected to periods of bright light stimulus. The results, according to senior author Diane Boivin, demonstrated that exposure to even normal indoor light can reset the human body clock by several hours.
These results are said to occur because of the suppression of melatonin secretion by ocular stimulation with light. The latter, a hormone secreted by the pineal gland, depresses body temperature, facilitates sleep onset, and can advance or delay the circadian rhythm. Today, light therapy is being used for adjusting sleep patterns and alleviating depression among people in northern latitudes where winter brings shortened periods of daylight. Scott Campbell and Patricia Murphy from Cornell University Medical College recently created a stir among those engaged in chronobiological research by reporting that they could reset a person’s body clock by as much as three hours simply by shining light at the backs of their knees! How could this work? In an article in the same issue of Science that carried the original research, Dan Oren and Michael Terman makes the startling suggestion that the blood molecules hemoglobin and bilirubin may play the same light- sensitive role in humans that chlorophyll and phytochromes do in plants. They believe the molecules are somehow light-activated while the blood is in the region of the knees and later stimulates the pineal gland to decrease melatonin production upon reaching the brain. There is no known mechanism by which this might take place.
Anyone who suffers from the winter blahs knows the duration of daylight can significantly affect a person’s mood. Treating mild depression with light is only one of the many ways that the body’s natural rhythms are being used to affect its well-being. Chronotherapeutics is the practice of administering treatment at precise points in the biological cycle in order to achieve maximum effectiveness. At a 1996 American Medical Association press conference Dr. Michael Smolensky, Director of the Hermann Center for Chronobiology and Chronotherapeutics said that physicians are now doing more than prescribing the right drug. “They are also delivering drugs at the right time in order to be optimally effective and safe,” said Smolensky. Among the ailments that are tied to the body’s circadian cycle are heart disease, asthma, migraine headache, and high blood pressure.
Clair Wood is the Bangor Daily News science columnist.