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February 09, 2006

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blue Lockley
Steven Lockley and his colleagues have found that blue light can cure the biological blues of shift workers, long-distance travelers, and others more effectively than extra exposure to white light. It even works for blind people. (Staff photos Maggie Mastricola/Harvard News Office)

When the blues keep you awake

Blue light can reset your biological rhythms

By William J. Cromie
Harvard News Office

Your eyes do more than see.

Researchers at Harvard Medical School demonstrated this by showing that your eyes are part of a light reception system that can keep you alert when sleep starts to fog your brain. When the researchers exposed people to blue light at night, this system immediately increased their alertness and performance on tests.

"These findings add to a growing body of evidence that a novel light reception system exists in the human eye in addition to sight," says Steven Lockley, an assistant professor at Harvard Medical School and a researcher in sleep medicine at Brigham and Women's Hospital in Boston. "Men and women exposed to blue light sustained a high level of alertness during the night when people feel most sleepy. These results suggest that light may be a powerful countermeasure for the negative effects of fatigue for people who work or study at night."

It's well known that white light can do the same. Long-distance travelers and shift workers sit in front of commercial light boxes that shift their natural clocks with glaring white light, hyping their attention to daylight levels. Apparently, blue light can do this more effectively.

It is even superior to green light, to which human vision is most sensitive. Sleepy people bathed in blue light for six and a half hours performed better than those lit by an equal amount of green light. They consistently rated themselves as less sleepy, showed quicker reaction times, and had fewer lapses of attention than those exposed to green light.

Lockley in lab
Steven Lockley goes blue to keep alert.

To Lockley and his colleagues, this is proof that eyes are not just used to see. Our orbs also monitor light for the purpose of setting our biological clocks to a 24-hour day. If clock setting and shifting were done by vision alone, it would be more sensitive to green, not blue, light.

Further proof comes from the responses of totally blind people. Most of these people toss and turn through a lifetime of sleeping problems because light cannot reach their brains to adjust their biological clock each day. But a small number easily adjust to the 24-hour cycle of light and darkness even though they cannot see it because their light detection system remains intact (see Sept. 25, 2003, Gazette).

Improving alertness

Lockley and his colleagues describe details of the light tests they performed on 16 healthy men and women in the Feb. 1 issue of a scientific journal appropriately called Sleep.

Co-authors included Charles Czeisler, a Harvard professor who studies the effects of light on daily body rhythms and sleep, and George Brainard, a neurologist at Thomas Jefferson University in Philadelphia who developed the specialized light equipment used in the experiment.

"The results open up a whole new range of possibilities for using light to improve human health," Lockley comments. "One application could be to improve alertness in people who need sustained attention for a long time, such as long-distance drivers, pilots, or even astronauts." Blue-lit shift workers might increase safety in dangerous situations and emergencies that arise on night shifts. Finally, properly timed blue light may ease the sleep hangovers of those jetting across time zones for business or pleasure.

People who already use white-light boxes to reset their biological clocks should be able to do so more efficiently with blue light, Lockley suggests. You can shift your sleep-wake cycle with less light energy or less time in front of a light box.

Asked if blue-light boxes are commercially available yet, Lockley noted that "many companies sell them. In theory, you could get the same effect in a room lit with ordinary blue bulbs, but this has not yet been studied."

How does blue light, or for that matter white light, change our 24-hour rhythms? That's not perfectly clear yet.

During the past few years, a specialized set of light-sensitive cells, which have nothing to do with vision, were discovered in the retina at the back of the eyes. These cells boast extensions that reach deep into the brain and connect with a small area of the hypothalamus that contains the workings of our internal clocks. What remains unknown is whether light affects this area directly or through a sleep hormone known as melatonin. During night hours, melatonin peaks, increasing sleepiness. As daylight comes, or during artificial exposure to light at night, this hormone veil lifts and enhances alertness. Light may also signal other parts of the hypothalamus, changing alertness without changing melatonin levels. Either way, the internal clocks' works are most sensitive to blue light.

SAD and safety

Other studies are under way on this and on other aspects of colored light. For example, researchers want to know how changes in the biological clock impact sleep on succeeding, blueless nights. If you stay awake longer or are more alert when awake, do you sleep longer and deeper when you finally go to bed?

Other researchers at Harvard Medical School and Brigham and Women's Hospital attempt to see how exposure to different colors of light can affect human health. One group studies whether or not sleep problems that plague older people can be solved by timed exposure to blue light.

Another team has begun an investigation of blue light to treat SAD, a seasonal affective disorder characterized by mood swings and other psychological changes that occur with the seasons. Feelings of sadness and depression during cold, dark winter months are an example.

A third group, under Czeisler, looks into using blue-enriched white light to shift the biological clocks of astronauts and their support staffs to days on Mars, which are longer than Earth days by about 36 minutes.

Because blue light contains more energy than white, concern exists that long-term exposure may damage the eyes. Too much blue light might cause problems like age-related macular degeneration, progressive damage to the retina common in older people. "This is a warning that we should not just use blue light without thinking carefully about the timing and duration of exposure and monitoring any routine exposure," Lockley says.

On the plus side, he continues, "developing 'smart' lighting systems that can be programmed to control color, duration, and pattern of exposure throughout the day and night could maximize the beneficial effects of light for human health."

Copyright 2007 by the President and Fellows of Harvard College