February 08, 1996
University Gazette


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Ordinary Room Light Resets Biological Clocks

By William J. Cromie

Gazette Staff

Brain and body rhythms can be reset to changes in the day-night cycle by ordinary indoor light, Harvard researchers have discovered.

Their findings upset a long-held belief that banks of bright lights are needed to synchronize biological rhythms to a 24-hour cycle of light and darkness. Getting out of synch causes jet lag and erodes the performance of shift workers.

"Our results clearly demonstrate that humans are much more sensitive to light than we previously believed," said Richard Knonauer, Gordon McKay Professor of Mechanical Engineering.

He worked with Medical School researchers at Brigham and Women's Hospital (BWH) testing the response of 31 male volunteers to varying light levels during experiments spread over the past four years.

"We found that as little as five hours of indoor light can reset the circadian [24-hour] pacemaker," noted Diane Boivin, a research fellow in medicine at BWH. This means that people could inadvertently readjust their biological rhythms, perhaps increasing sleepiness or wakefulness at inappropriate times.

"Shielding the morning light with shades and heavy curtains may cause a profound impact on the body's synchronization to a normal 24-hour day," said Charles Czeisler, associate professor of medicine and director of the circadian and sleep disorders laboratory at BWH.

The study brought to light a mathematical relationship between the intensity of light and the resetting of biorhythms. "Exposing men to five hours of bright light on three successive days can shift their biological clocks four to six hours," explained Kronauer. "A five-hour exposure to normal indoor lighting on three successive days produced a shift of about 1.5 hours. That's not insignificant."

Shift workers, international travelers, even those suffering winter depression can adjust their daily rhythm with commercially available light banks. Such devices typically shine about 50 times more light than is present in a normally illuminated room.

The experimenters excluded women from the study because light shifting can change menstrual cycles and confound research results. "Additional studies of the complex interaction between the circadian and menstrual cycles have already begun," Czeisler said.

"Right now we're satisfied with the discovery that exposure to ordinary indoor illumination can play a role in changing human circadian rhythms," Kronauer noted. "For those who live in urban areas, it is probably the principal synchronizer of biological rhythms to natural changes in day and night."

Boivan, Kronauer, Czeisler, and Jeanne Duffy of Northeastern University report details of this research in today's issue of the British journal Nature.


Copyright 1998 President and Fellows of Harvard College