Campus & Community

When light has you singing the blues:

6 min read

Blue light resets body rhythms for sounder sleep, higher alertness

Steven Lockley and his colleagues found that blue light is more efficient at resetting biological clocks than standard white light, a discovery that could help jet-setters, shift workers, astronauts, students, and military personnel. (Staff photo Kris Snibbe/Harvard News Office)

Blue light outshone white in a Harvard University experiment to find better ways to reset our body clocks.

Jet-setters and shift workers now sit in front of glaring white lights to readjust their body rhythms and avoid sleep and alertness problems. These new experiments suggest that they would be better off mugging for blue lights.

The research also contradicts what many scientists believed for years, that the 24-hour biological clock is set by sight alone. Until 1995, dogma held that the intensity of light striking receptors that give humans color vision also adjust the daily cycle that controls sleep, performance, and other physical and behavioral factors. Now, there is conclusive evidence for a second system that dominates the setting of daily rhythms in creatures from bacteria to international travelers, even blind ones.

“The visual system in humans is most sensitive to green light,” notes Steven Lockley of Brigham and Women’s Hospital, a Harvard research and teaching affiliate. “But when we exposed 12 healthy young men and women to the same amount of either green or blue light, their 24-hour rhythms shifted twice as much with blue than with green.”

After 6.5 hours of exposure, blue light readjusted their body clock by 3 hours, green light by about 1.5 hours. If you normally feel your eyelids getting heavy at 11 p.m., a 6.5-hour dose of blue light might keep you alert until 2 a.m. If a change in shift leaves you sleepless at 4 a.m., blue light might help you sleep three hours longer. (Which way the shift goes depends on when the light exposure takes place.)

To explain the research findings, there has to be one system that controls color vision, providing a greater acuity for green than for other colors, or wavelengths, of light. A separate system must involve a 24-hour pacemaker that keeps your body linked to the daily ups and downs of the sun.

“Both systems might be involved in setting circadian (24-hour) rhythms, but we think that the one that responds to blue light is the dominant one,” says Charles Czeisler, a professor of medicine at Harvard Medical School, in whose laboratory the research takes place. “The wider-ranging implication of our work is the demonstration that the standard of illumination used by the lighting industry and clinical research community is inappropriate when assessing its effects on the circadian system.”

“Those implications,” adds Lockley, “extend to the design of light systems to treat circadian sleep disorders such as those tied to shift work, jet lag, and long-duration space travel. They also bear on changes in light exposure associated with aging and blindness, as well as ensuring proper alignment of internal rhythms with the outside world, particularly important in the current 24/7 society.”

The blind lead the sighted

The obvious way to prove that separate systems govern color vision and body rhythms is to test blind people. Czeisler was trying to puzzle out why some blind people easily adjust to the 24-hour cycle while others toss and turn through a lifetime of sleeping problems, when he found the proof.

In 1990, he interviewed a blind man who claimed to experience no trouble sleeping. “I thought to myself that he is either not totally blind or he’s not aware that his sleep is disturbed,” Czeisler recalled. “Some blind people are not aware that their sleeping patterns differ from those of sighted people. Eventually he convinced me he had no trouble sleeping.”

Moreover, his light blue eyes were “crystal clear and healthy looking. That led me to wonder whether his blind eyes might still be able to convey photic (light) information to the circadian clock.”

Czeisler then checked to see if exposure to light would suppress a hormone known as melatonin. During night hours, melatonin peaks, decreasing alertness and increasing sleepiness. Daylight clears away the veil as melatonin levels decrease. That’s what happened in his blind subject.

If the man couldn’t see, something else must be keeping his body clock running smoothly. “It was a ‘eureka moment’ for me,” Czeisler remembers. “It changed my worldview of how light resets the human clock.”

But it didn’t change the world’s view. Czeisler reported his discovery but no scientific journals would publish his work. Other researchers did similar experiments on blind rats and mice, and they got the same results. The evidence became too overwhelming to ignore, and the discovery that human eyes have two functions was finally published in the New England Journal of Medicine in 1995.

Setting the blues

The recent experiments with blue light, published in the Journal of Clinical Endocrinology and Metabolism this month, add further proof that eyes both see and adjust internal body rhythms.

How do the clock adjustments take place? A specialized subset of light-sensitive cells, which apparently have nothing to do with vision, exists in the retina, or back of the eye. These cells boast extensions that reach deep into the brain to the hypothalamus, the location of the body’s internal clock. They contain melanopsin, a recently discovered substance that may sense light and transmit signals that change the clock settings.

“That’s a theory,” Czeisler cautions, “not a done deal. The melanopsin cells may operate alone or it may get input from the classical vision system. In any case, melanopsin seems to dominate.”

“About 20 percent of totally blind people have their biological clocks synchronized by light even though they cannot see it,” Lockley points out. “Of the other 80 percent, more than half suffer serious sleeping problems that result from a failure to reset the clock.”

Blind or not, why does blue light set the clock more effectively than green or red light? The short answer is that no one knows. “But it’s probably not an accident that looking up at the blue sky has twice the resetting effect on our circadian clocks as looking down at green grass,” Czeisler notes.

One theory holds that the birth of life on Earth occurred in the ocean, and that origin established an evolutionary preference. The idea is backed up by evidence that some of the earliest creatures to evolve, one-celled algae, react preferably to blue light.

Lockley, Czeisler, and George Brainard, a researcher from Thomas Jefferson University in Philadelphia who collaborated on the experiments, readily admit there are a lot of loose ends to tie up before the mysteries of internal pacemakers are solved. One thing they want to do next is to determine how much easier it would be to use blue light for resetting the alertness of jet travelers, shift workers, students, astronauts, and the military. Czeisler imagines a widely available blue-light system in homes that would adjust your body clock continuously to your schedule.

“Our study opens the door to both understanding how humans and other organisms adjust to the planet’s rhythms, and how we can practically align our internal time to the demands of a 24/7 society.”