Green Light Affects Circadian Rhythms

Researchers show that green light is effective in eliciting non-visual responses to light such as resetting circadian rhythms, affecting melatonin production and alerting the brain.

Online PR News – 11-June-2010 – – It has been previously shown that blue light plays an important role in impacting the body's natural internal body clock and the release of hormones such as melatonin, which is connected to sleepiness, by affecting photoreceptors in specialized cells in the eye. In new research from Brigham and Women's Hospital (BWH), researchers have found that green light also plays a role in influencing these non-visual responses. This research is published in the May 12 issue of Science Translational Medicine.

"Over the past decade there have been many non-FDA approved devices and technologies marketed for using blue light therapeutically such as blue light boxes for treatment of Seasonal Affective Disorder and circadian rhythm sleep disorders, and glasses that block blue light from reaching the eye," said Steven Lockley, PhD, a researcher in the Division of Sleep Medicine at BWH and senior author of the paper. "Our results suggest that we have to consider not only blue light when predicting the effects of light on our circadian rhythms, hormones and alertness, but also other visible wavelengths such as green light."

In the human eye there is a novel photoreceptor system, separate from the rods and cones used for vision, which detects light and is responsible for non-visual responses, such as resetting the internal circadian body clock, suppressing melatonin release and alerting the brain. These photoreceptors are located in the ganglion cell layer, where there is a specialized subset of cells which are specifically responsive to blue light. Previous research has indicated that these cells are the primary way in which light is detected for the non-visual responses. This new research from BWH shows that cone photoreceptors, which are used for color vision and are most sensitive to green light, also play a role in eliciting non-visual responses.

Researchers enrolled 52 healthy volunteers in a 9-day study in the Intensive Physiological Monitoring unit at BWH, which is free from all time cues including windows, clocks, internet, television or any other indicators for the time of day. Researchers shifted the participants' schedules so that they slept during the day and were awake in the night, during which time they were exposed to 6.5 hours of either green or blue light to simulate an overnight work shift. The light exposure was timed to reset the internal circadian body clock later than normal, equal to the adaption required to prevent jet lag following a westward flight across time zones.

Researchers measured the effect of the light exposure on melatonin levels and the shift in the timing of the circadian clock. They found that while blue-light is usually the most effective way to stimulate the non-visual responses - especially under bright light conditions- stimulation with green light was also capable of eliciting the non-visual responses under certain circumstances. At the start of the light exposure or when exposed to dim light, green light was equally as effective as blue light at stimulating these non-visual effects, but then the effects died off more quickly over time.

"Our findings suggest that by dynamically manipulating the color, duration and pattern of light, current available light therapies could be optimized and new therapies could be developed. These findings have the potential to play an important treatment role for a number of disorders including circadian rhythm sleep disorders, seasonal affective disorder and dementia. They could also be applied to the use of light as a drowsiness countermeasure, particularly during night shift work, or anywhere were sleepiness might affect performance or present a safety concern" Lockley said.

This study was funded by the National Institutes of Health and the National Space Biomedical Research Institute.

Brigham and Women's Hospital (BWH) is a 777-bed nonprofit teaching affiliate of Harvard Medical School and a founding member of Partners HealthCare, an integrated health care delivery network. In July of 2008, the hospital opened the Carl J. and Ruth Shapiro Cardiovascular Center, the most advanced center of its kind. BWH is committed to excellence in patient care with expertise in virtually every specialty of medicine and surgery. The BWH medical preeminence dates back to 1832, and today that rich history in clinical care is coupled with its national leadership in quality improvement and patient safety initiatives and its dedication to educating and training the next generation of health care professionals. Through investigation and discovery conducted at its Biomedical Research Institute (BRI), BWH is an international leader in basic, clinical and translational research on human diseases, involving more than 860 physician-investigators and renowned biomedical scientists and faculty supported by more than $416 M in funding. BWH is also home to major landmark epidemiologic population studies, including the Nurses' and Physicians' Health Studies and the Women's Health Initiative. For more information about BWH, please visit

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