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Light studies focus on circadian rhythms

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Rebecca C. Jernigan,

Daylight, or the lack thereof, has a dramatic effect on our lives: Gloomy days tend to make us sleepy, while sunny days wake us up. A lack of sunlight during the dark winter months is believed to cause seasonal affective disorder or depression.

Investigators at the Lighting Research Center at Rensselaer Polytechnic Institute, led by Mariana Figueiro, director of the center’s Light and Health Program, have been working to determine how and why these effects occur and whether lighting can be used to help various segments of the population to function more effectively. Using the Daysimeter, a device they developed a few years ago, the scientists have been tracking light exposure and activity levels in volunteers in middle schools, nursing homes and hospitals to find correlations among them and to determine levels of circadian entrainment and disruption.

A researcher adjusts EEG electrodes on a subject undergoing exposure to blue light.

The Daysimeter is a small head-mounted device that incorporates both a photopic and a blue light sensor. The former measures the amount of visible light the device is exposed to, while the latter tracks short-wavelength light, which is known to have an effect on the circadian system that is responsible for regulating the wake/sleep cycle. The Daysimeter also has an actigraph – a circuit board that collects information about the subject’s rest and activity patterns by tracking head movements and measuring their g-force. The device, which can be worn for up to 30 days without recharging the batteries, is small enough to be worn as a headset.

Figueiro said that the design yields more accurate information than previous versions, which were also larger, because it tracks the light at eye level, ensuring that the measurements match what the user actually experiences. With support from a Trans-National Institutes of Health Genes, Environment and Health Initiative grant, the researchers hope to shrink the device further, possibly miniaturizing it to the size of an earring. They also plan to expand the range of data it gathers to include heart rate and tympanic temperature.

The Daysimeter, shown on a volunteer, measures the wearer’s daily rest and activity patterns, as well as exposure to circadian light. Images courtesy of Lighting Research Center.

Multiple studies have been performed using this device. In mid-2008, the team worked with Harvard Public Health on a project sponsored by the National Institutes of Health to investigate the impact of circadian disruption in nurses working either the day shift or a rotating shift. Participants wore the Daysimeter for one week while going through their daily routines. Simultaneously, the researchers studied the effect of irregular light exposure on the circadian system of 40 rats to determine whether the relationship between circadian disruption and health outcomes could be revealed using rodent models.

The results, detailed in the May 29, 2008, issue of the Journal of Circadian Rhythms, show that the circadian entrainment and disruption patterns for day-shift and rotating-shift nurses were remarkably different from each other but remarkably similar to the patterns for the two parallel groups of nocturnal rodents. The ability to quantitatively define circadian light and dark for humans as well as nocturnal rodents will allow researchers, using rats as test subjects, to carry out studies relevant to humans. The study’s findings will help determine the effect of light exposure on various human health problems, including breast cancer, obesity and sleep disorders.

The Daysimeter is also being used to investigate the impact of light exposure on activity levels of middle-school children, with the intent of developing in-school lighting systems that help regulate the students’ circadian rhythms. This project, supported by a $250,000 grant from the US Green Building Council, a nonprofit trade organization, will compare the activity patterns and light exposure of students in schools with “good” and “bad” daylighting designs (which use natural light to illuminate the interior of a building). The study is monitoring the children’s sleep patterns, levels of psychosocial stress and psychomotor vigilance. A preliminary study investigating the importance of school lighting has been completed, and a paper describing the results is being written.

Scientists at the institute are working on several other projects, which are in various stages ranging from data collection to waiting for a paper to be published, with much of the information expected to be available this fall.

Jul 2009
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
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