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
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
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
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
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
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
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
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