A SAD story: Seasonal affective disorder

Light therapy and antidepressants help people who get depressed during the winter.

The gloom of winter seems to get inside some people, the dark affecting their mood as well as their days. In the late 1990s, the American Psychiatric Association (APA) recognized these winter blues as seasonal affective disorder, a name that seems to have been coined with its acronym, SAD, very much in mind.

Light therapy, which involves sitting in front of bright, artificial light for a half an hour or so each day, lifts the mood of some people who suffer from the condition. But antidepressant medications may work just as well. There's also evidence, some of it anecdotal, that people with SAD will feel better if they get outside more, open up window shades, brighten up their indoor living spaces — in sum, take steps to increase their exposure to light over all.

The same may be true for the many of us who don't meet the criteria for a SAD diagnosis, but who still get blue during these short days of winter.

Feel like hibernating?

People troubled by depression usually experience their dark moods in an on-again, off-again fashion. In that respect, SAD differs only in that the oscillations follow a seasonal schedule, with the depression usually coming on in the fall and lasting through the spring. There is a summertime version of the disorder, but it affects far fewer people.

SAD sufferers get depressed at other times of the year, but the APA's definition requires that the seasonal episodes "substantially outnumber" the nonseasonal ones. And, strictly speaking, the diagnosis requires two straight years without any out-of-season depressions. Psychiatrists also face the tricky task of separating depression that might be triggered by events that happen regularly in the fall and early winter — the start of school, seasonal unemployment — from depression associated with the season itself.

The winter version of SAD is often compared to hibernation. People lack energy, sleep more than usual, crave sweets and starchy food, and gain weight. They may withdraw socially and have a hard time concentrating. Some describe a heavy feeling in the arms and legs. A condition called atypical depression, which is not associated with the seasons, has similar symptoms.

Out of sync

Experts debate whether a causal relationship has been proved, but there's certainly circumstantial evidence that lack of sunlight in winter triggers SAD. The fact that light therapy is a remedy is one clue to a connection. Epidemiologic evidence showing that SAD is more common and lasts longer among people living at high latitudes is another.

Dr. Norman E. Rosenthal, a psychiatrist at Georgetown University Medical Center who was among the first to describe seasonal affective disorder, believes that the root cause may be insensitivity to light. Most of us go through winter on a relatively even keel because exposure to indoor lighting helps offset the lack of natural light, explains Dr. Rosenthal, author of Winter Blues: Everything You Need to Know to Beat Seasonal Affective Disorder (Guilford Press, 2005). But indoor light may be too weak for SAD sufferers.

Exactly how lack of light can influence our moods is an open question. Light, of course, is the stimulus for vision: it excites cells in the retina in the back of the eye that send signals to the areas of the brain that create and process visual thought. But there are neural pathways from the retina to other parts of the brain, including the suprachiasmatic nucleus, a part of the hypothalamus that helps put many of our physiological processes (sleeping, body temperature) on a circadian, or 24-hour, cycle (see illustration).

Seasonal affective disorder: Brains that don't like it dark

So, according to one theory, people with SAD are out of phase with their biological clocks: awake and active when their internal timers, guided by the suprachiasmatic nucleus, would rather that they be snug in bed.

The suprachiasmatic nucleus also has a hand in regulating melatonin, a hormone that's secreted by the pineal gland when it gets dark. It's possible that SAD sufferers overproduce melatonin in winter, or become hypersensitive to its effects.

Another theory: a lack of, or insensitivity to, light disrupts brain processes influenced by serotonin and dopamine, brain chemicals that play a role in setting our mood.

It's also possible that other parts of the brain and body are involved in biological timekeeping, so even if the suprachiasmatic nucleus and related structures get in sync with winter, they're overruled, so people remain out of phase.

Lighten up

Studies of light therapy for SAD go back to the 1980s, but many have been small, short, or flawed in significant ways. Researchers conducting a meta-analysis found only 20 studies of the 64 published met their standards. Even careful investigators run up against the problem of what to use as a placebo. Still, there's enough wheat amid the chaff to safely say that light therapy does help some people with SAD. Most studies have found it to be more effective first thing in the morning than later in the day.

Some SAD light boxes look like medical equipment, while others are more like regular table lamps. The prices vary, but from our quick look online, it seems you should expect to spend in the range of $100 to $200. Professional groups and government agencies have endorsed light therapy, but your insurance company may balk. If you are counting on coverage, better check first.

You can start gradually, but most studies suggest SAD sufferers need about 30 to 45 minutes of exposure to a 10,000-lux light source daily for light therapy to be effective. Lux is a measure of light intensity; 10,000 is about the intensity of light from the sun at dawn. In most homes in the evening, 300- to 500-lux light is the norm.

Exposure to such bright light has caused some concern about eye damage. Some companies sell light boxes that don't emit light in the blue part of the spectrum, because blue light is believed to be more damaging to the retina. Others sell equipment that uses LED lights that can be calibrated to emit light in the range of the spectrum that the companies contend is more effective at resetting circadian rhythms.

Although the chance of eye damage can't be entirely ruled out, the studies published so far haven't found there to be great cause for concern. Most experts say regular fluorescent lights — as long as they have an ultraviolet (UV) filter — are safe and effective. The side effects include headaches and eyestrain. And there have been scattered reports of light exposure triggering mild manic episodes (hypomania).

Light therapy has been tested as a treatment for several other conditions, including bulimia nervosa (which sometimes overlaps with SAD), postpartum depression, and Parkinson's disease. We've posted some information about that research on our Web site at www.health.harvard.edu/health.

Pills instead

Light therapy takes time and some discipline: people have to sit in front of those lights pretty much every day. Antidepressant medication would be a lot more convenient. In 2006 the FDA approved Wellbutrin, a brand of bupropion, as a treatment for SAD. The approval was based on several studies showing that the drug worked to prevent SAD symptoms if people started taking it in the fall before symptoms started.

A number of studies have shown that fluoxetine (Prozac), sertraline (Zoloft), and other drugs in the selective serotonin reuptake inhibitor (SSRI) class of antidepressants might help SAD patients. Canadian researchers reported trial results in 2006 that showed fluoxetine and light therapy were equally effective (a 67% response rate).

People take melatonin for jet lag, but it might be useful for SAD, too. Oregon researchers have reported that taking melatonin pills in the afternoon helped some SAD patients by resetting their biological clocks.

'More light'

If light is the key to relieving SAD, why sit indoors in front of an artificial version? That seemed to be the question that Swiss researchers were asking in the 1990s when they designed a trial that compared a one-hour walk each morning to sitting in front of a light box for half an hour. The mood of the walkers improved much more than that of the sitters. Perhaps it was the physical activity — not the exposure to outdoor light — that brightened their outlook. And the 2,800-lux light used in the study is far less intense than the 10,000-lux light used today.

Those who believe SAD is related to problems of circadian rhythm think that the trick is to reset the biological clock with light in the morning. Still, the Swiss study is a reminder that light exposure at other times of the day might help some SAD sufferers, perhaps especially those with mild cases. In his book, Dr. Rosenthal discusses patients who have benefited from interior decorating (more lamps and ceiling lights) and winter travel, even to Antarctica, where it's very sunny indeed during the Northern Hemisphere's winter.

The story is that "more light!" were among the last words uttered by Goethe, the German writer and author of, among other works, The Sorrows of Young Werther. He meant it as a request, but perhaps it's good advice for getting through the winter.