During a 24-hour period humans experience a rise and dip in the production of most hormones and neurotransmitters (the chemicals that relay signals between nerve cells). This daily cycle is referred to as the body's circadian rhythm and is regulated by both internal systems and external stimuli, the most powerful of which is visible light.

In an effort to gauge exactly how light affects our body clocks, Rensselaer Polytechnic Institute's Lighting Research Center (LRC) in Troy, N.Y., has developed a device called a Daysimeter. Small and ear-mounted—like a wireless cell phone headset—it has three sensors that measure head movement, bright light (or lux, a measurement of the light used for daytime vision), and blue visible light (also known as circadian light). Circadian light—radiated by the sun as well as computer and television screens—helps balance certain hormones and neurotransmitters in the body, but only in specific doses and at certain times of day. Too much of this light can throw off the body's internal clock, which researchers believe leads to problems such as fatigue and poor health.

Researchers at the LRC, in collaboration with Yale and Brown universities, last month began testing the device on 24 Brown students to see if they can modify a person's circadian rhythm. The volunteers were instructed to wear the wireless instrument throughout their daily routines.

"We envision the Daysimeter, along with other biological markers [such as hormones] will allow us to get a more detailed circadian profile of a particular person," says LRC director Mark Rea, a Rensselaer professor of cognitive science. Researchers can measure the effect of circadian light exposure on hormone levels through blood samples collected from subjects. "We're fully expecting that we'll see variation among the population," he notes.

Rea envisions "real-time light prescriptions" to help people receive or avoid light at the appropriate times. Simple measures to control when and how much circadian light we receive could help nightshift workers stay alert on the job and sleep more effectively during the day, help cure jet lag, decrease depression, and generally help everyone get a proper night's sleep.

The ability to modify circadian rhythm could potentially mitigate the negative health effects that some researchers believe are brought on by disruptions to the light-dark cycle. Recent studies have found a link between health and changes in the natural circadian rhythm. The Journal of the National Cancer Institute published a series of articles, for example, that showed night shift workers had a higher incidence of breast cancer; and, last year, the World Health Organization's International Agency for Research on Cancer cited night work as a potential breast cancer risk factor.

"Light is the most important environmental signal that tells our body what time of day it is," says Steven Lockley, assistant professor of medicine at Harvard Medical School's Division of Sleep Medicine at Brigham and Women's Hospital in Boston. With the advent of electricity came exposure to artificial light at night, triggering changes in the normal light-dark cycle and upsetting the body's circadian rhythm. "If we use artificial light at a time when the external light is gone, during the night," Lockley says, "then we start to confuse our body clocks."

Measuring and modifying circadian rhythm builds on discoveries made over the past several years. In 2001 George "Bud" Brainard, a neurology professor at Thomas Jefferson University in Philadelphia, identified the wavelength of circadian light, and Josephine Arendt, founder of the University of Surrey's Center for Chronobiology in Guildford, England, discovered that eyes detect circadian light through different photoreceptors than those used for optical vision. In 2002 Brown University neuroscience professor David Berson and biologist Ignacio Provencio of the University of Virginia identified melanopsin, found in the retina's light-sensitive ganglion nerve cells, as the photoreceptor for circadian light. Lockley discovered that these photoreceptors function even in blind people.

Rea admits the LRC is a long way off from making their instrument available to the public. But researchers in the science of circadian rhythm are excited by the prospect of devices that may one day help people understand their own particular light-dark cycle and how to keep it in balance.