LOW LIGHT SHIFTS BODY'S RHYTHMS, STUDY FINDS.

Byline: Warren E. Leary The New York Times

Researchers say they have found that normal levels of indoor lighting, and not just very bright light, can reset the human biological clock, a finding they say indicates that many people in industrialized countries may be constantly sleep deprived and in a permanent state of jet lag.

A five-year study of human response to light in the ranges found in modern homes and offices indicates that the modern shifting of night to day with artificial lighting may have profound effects that were previously unsuspected.

"We think Thomas Edison had a bigger effect on the human body clock than anyone realized," the senior researcher, Dr. Charles A. Czeisler of Brigham and Women's Hospital in Boston, said Wednesday.

The widespread availability of electricity and light bulbs has provided most people with enough extra light exposure each day to significantly shift the timing of their internal clocks, which govern sleep and restfulness, among other things, he said in an interview.

Since early research showed that exposure to bright light could reset a body clock that had gone awry, Czeisler said, many scientists argued that an intense light was required. But he said the new research indicated that while bright light produces intense, rapid changes in body clock timing, long exposure to much lower levels of light could have more profound effects over time.

In a report being published today in the journal Nature, Czeisler, Dr. Diane B. Boivin and colleagues said that exposure to relatively low-light intensity from artificial lamps for as little as five hours could reset the biological clock, also known as the circadian pacemaker.

"Our results clearly demonstrate that humans are much more sensitive to light than initially suspected," they said. They added that their work also supported the conclusion that people were not qualitatively different from other mammals in having their circadian rhythm reset by low levels of light.

Dr. Thomas A. Wehr, chief of the clinical psychobiology branch of the National Institute of Mental Health in Bethesda, Md., said the Boston study was the latest work to show the influence of low-level light on body rhythms. Wehr concluded in a recent study that artificial light suppressed the ability of men, but not women, to distinguish between summer and winter daylight patterns.

"Bright light is still important because it affects the biological clock more rapidly and intensely than ambient light," Wehr said in an interview, referring to indoor lighting. "Bright light causes a big shift in a short period and may be more useful in treating jet lag and the effects of shift work, where you want a quick response. Lower levels of light might have a therapeutic use if manipulated over a period of time."