For the 3% of Americans whose depression is seasonal, the problem boils down to a shortage of light. Not just any old light, but light that ends the body's internal night.
The most specific treatment for seasonal affective disorder (SAD) is exposure to artificial light of sufficient intensity and character to shut off the body's nightly production of melatonin. For most people, the arrival of morning suffices to accomplish the task; but for others a light boost is needed because as the seasons change, the lessening hours of daylight fail to do the job on their own.
Light therapy is at least as effective for SAD as drug therapy is for major depression. Upwards of 65% of patients experience marked relief of symptoms, and most do so within days of starting treatment.
Developed in the 1980s, light therapy has been extensively validated since. And while guidelines spell out how light therapy is best administered and dosage required to produce results, the precise type of light and exposure needed for maximal biologic effect are constantly being refined.
Researchers have known for some time that full-spectrum light is most effective against SAD. But only recently, for example, have they pinpointed the portion of the spectrum that most efficiently suppresses nighttime melatonin secretion.
According to Dr. George C. Brainard and colleagues at Thomas Jefferson University, it is light with the shortest wavelength, in the blue visible range, that has the most potent input for regulating melatonin. His studies suggest that the human eye contains unique photoreceptors dedicated to regulating the body's master clock of circadian rhythms.
And these photoreceptors are most sensitive to light that travels at wavelengths of 446 to 477 nanometers. The visible spectrum begins at around 400 nm, with cool white light having a high proportion of blue in it. Light under 400 nm in wavelength is ultraviolet light, not visible to the eye but capable of damaging it. For this reason, it is essential that light devices utilizing full-spectrum fluorescent source incorporate a filter to screen out any ultraviolet rays.
In addition to wavelength, the dose of light needed for biologic effects depends on several other factors:
Intensity. Usually expressed in lux, intensity is a measure of the illuminating power of light. Typical living room evening light is less than 100 lux; bright office lighting is usually 300 to 500 lux, outdoors on a cloudy day measures 1,000 to 5,000 lux. Direct midday sun can top 50,000 lux.
Research has shown that bright light of at least 2,000 lux is needed for biologic effect. Until recently, a fluorescent light box emitting 10,000 lux was the benchmark for SAD therapy, but new research suggests that light of 2,500-3,000 lux may work well.
The amount of light that actually reaches the photoreceptors, however, is not just a function of light emitted. The dose of light a person receives depends on where he or she sits, as light level drops precipitously with increasing distance from the source. As a result, most manufacturers of light devices specify exactly how far from the unit the user should sit, usually around two feet.
Duration of exposure. Light treatment should be used daily in winter, starting in early autumn when the first symptoms appear. Clinical reality suggests starting with once-a-day-dosing of 30 to 60 minutes. Some people may need more, others less. Every person is unique and needs to experiment with finding the duration of exposure that works best.
Timing of light exposure. There is clear consensus that early morning light is most effective in suppressing melatonin. "The best time for light therapy is as soon as someone wakes up," says Dr. Thomas Wehr of the National Institute of Mental Health. It isn't necessary to stare at the light for it to reach the target photoreceptors. Users can carry out such everyday tasks as eating, reading and writing.
There are many types and models of light devices available for SAD treatment. There are light boxes that sit on desks or that can be mounted on a wall. There are floor lamps and bedside dawn simulators. There is discreet hand-held model and a portable device no bigger than a CD player. There's even a compact device fitted into a visor. There are devices that employ fluorescent lighting and others that utilize light emitting diode (LED) technology. Whichever you choose, it is important that the device be covered by a warranty from a quality manufacturer. Some leading sources, with their toll-free numbers, include:
Northern Light 1-800-263-0066
UPLIFT Technologies 1-800-387-0896