New Light on Seasonal Depression

For 11 million people in the U.S. the cool crisp days of fall don’t bring a renewal of energy but rather a descent into depression. The body slows down. They feel mental and physical fatigue. They crave carbohydrates and gain weight. They’re slow to wake in the morning—a faint echo of seasonal patterns of response among other animals. But for them the change in energy level is so extreme they have trouble functioning.

The problem is probably as old as patterns of human migration to latitudes distant from the equator. Seasonal affective disorder, as it’s known, or SAD, is rare in those living within 30 degrees of the equator, where daylight hours are long, constant, and extremely bright.

But for the 11 million Americans, fall brings a collision of two vulnerabilities—one to depression, the other to seasonality. The problem is set off by a shortage of light. As the seasons change and days shorten, they do not get enough light to end the body’s internal night.

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Consequently come morning and their bodies fail to shut off production of melatonin, a hormone produced in the pineal gland that has a slowing effect on the nervous system. This biological signal of lengthening nights sets off a cascade of responses resulting in depression.

While normal people also experience a seasonal increase in melatonin production, they do not become depressed in response to it. SAD sufferers do, and their depression lifts in spring, when longer days signal their bodies to shut off melatonin in the morning.

The best and most specific treatment for SAD is substantial early-morning exposure to full-spectrum light. But a Philadelphia researcher now finds that the body’s biological clock is most sensitive to the blue portion of the light spectrum His findings promise not only to dramatically improve treatment for those who suffer from seasonal affective disorder but to revolutionize indoor lighting for everyone.

According to George Brainard, Ph.D., professor of neurology at Jefferson Medical College, the body’s internal clock is most sensitive to short-wavelength blue light. Previously he and colleagues had found that blue light is more effective than other light in halting body production of melatonin,.

Because melatonin production affects circadian rhythm, the researchers set out to test the effect of blue and green light on sleep patterns and other physiological and behavioral functions. Sixteen healthy subjects lived in a lab for nine days, where they received a daily dose of 6.5 hours of highly concentrated blue or green light.

Remarkably, blue light was twice as effective as the same amount of green light at resetting the subjects’ biological clocks, the team recently reported in the Journal of Clinical Endocrinology and Metabolism. The study proves that the eye is home not only of the visual system but also a separate system of receptors that runs on light to keep circadian time. “There exists a second, non-visual photoreceptor system that drives the body’s internal clock,” Brainard says.

He expects that the discovery will have an immediate impact on the therapeutic use of light for treating winter depression and circadian disorders. “Some makers of light therapy equipment are developing prototypes with enhanced blue light stimuli.”

Using only the more potent blue wavelengths will allow for the development of more effective, more portable and more tolerable devices. It will also significantly cut down the amount of light exposure needed.

Currently, SAD sufferers who expose themselves to full-spectrum white light for at least a half hour every day are more than 65% likely to experience relief of their symptoms, often within a few days. However, says Brainard, the intensity required, about 10,000 lux, is uncomfortable to look at and often causes glare headaches. Nor are all light boxes portable, posing treatment difficulties for frequent travelers.

Trials are now underway to test the therapeutic effect of blue light on SAD patients.

Brainard also predicts that the research findings will usher in a revolution in architectural lighting. “Home and work lighting could be optimally regulated for circadian effects,” he says.

“For thousands of years, man has been active during the day and restful at night. In modern times, our indoor environment is much dimmer and has a different pattern than daylight. In contrast, we have light on at night in our homes. In the future, lighting could accommodate not only the visual system, but the secondary system that regulates our biological clock as well.”

This is good news for everyone. For all of us, light is a powerful synchronizer of circadian rhythms, and we now know that the brain contains specific nerve pathways for daylight to shut off melatonin production.

It is especially good news for those who suffer from seasonal affective disorder, for whom light is a specific remedy that turns off nightly melatonin secretion. It’s also good news for all those jet-lagged travelers and the nearly 40 million Americans who suffer from sleep disorders.

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