"These findings add to the body of evidence that illustrates that there is a novel photoreceptor system that exists in the human eye in addition to that used for sight," said Steven Lockley, PhD, lead author of the study and a researcher in BWH's Division of Sleep Medicine. "Light exposure to this system, particularly blue light, directly reduces sleepiness. Subjects exposed to blue light were able to sustain a high level of alertness during the night when people usually feel most sleepy, and these results suggest that light may be a powerful countermeasure for the negative effects of fatigue for people who work at night."

In order to determine which wavelengths of light were most effective in warding off fatigue, the BWH researchers teamed with George Brainard, Ph.D, professor of neurology at Thomas Jefferson University in Pennsylvania who developed the specialized light equipment used in the study. They compared the effects of blue light (460 nanometers, nm) with exposure to an equal amount of green light (555 nm) on alertness and performance for six and a half hours during the night.

Subjects rated how sleepy they felt on a scale from one to 9, had their reaction times measured and wore electrodes to assess changes in brain activity patterns during the light exposure. The subjects exposed to blue light consistently rated themselves less sleepy, had quicker reaction times, and had fewer lapses of attention during the performance tests compared to those who were exposed to green light.

They also had changes in their brain activity patterns that indicated a more alert state. The finding that short-wavelength blue light was more effective indicates that the color vision system is not the primary photoreceptor system used to detect light for these effects as the visual system is most sensitive to the green light exposure used (555 nm).

Until recently, it was thought that the eye was just used to see. BWH researchers and others have shown that it is also used to detect light for other purposes, such as resetting the body clock to the 24-hour day. This photoreceptor system is different from that used in normal vision as it has a different sensitivity to the color of light and is retained in some totally blind people.

"These results have opened up a whole new range of possibilities for using light to improve human health. One application of these studies may be to help improve alertness in those people who need to sustain attention for long periods of time such as long distance drivers, pilots, or even astronauts. Blue light exposure for shift-workers could also improve safety in potentially dangerous situations that may arise due to the sleepiness experienced when working at night" said Lockley, who is also an Assistant Professor at Harvard Medical School (HMS).

He continues, "The therapeutic effects of blue light still need further investigation particularly with regard to the safety of long-term exposure, however. Blue light, if misused, can cause damage to the eye and exposures need to be carefully monitored. With the advent of new, more controllable lighting technologies, we can begin to develop 'smart' lighting systems designed to maximize the beneficial effects of light for human health."

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