Visitors sit in the Light Lounge, an ambient white space containing four specially designed light boxes where visitors can have light therapy, in the Science Museum's Dana Centre in London. Light therapy is used to help combat seasonal affective disorder, SAD. (John D. McHugh/AFP/Getty Images)

Winter is here. For some people, that means the winter blues are also here.

Officially called seasonal affective disorder — with its descriptive acronym, SAD — winter blues can leave people feeling low on energy and muted to the normal joys of life. The disorder is a form of depression whose symptoms include feelings of hopelessness, worthlessness and guilt, and even suicidal thoughts. The defining feature of SAD is its seasonality: It generally doesn’t occur in the summer.

SAD can be treated with talk therapy and/or drugs; Wellbutrin and Prozac are approved by the Food and Drug Administration for this use, though doctors can prescribe other medications off-label.

In addition, SAD can be treated with light therapy. This involves sitting near a very bright (10,000 lux) broad-spectrum light for 30 minutes or so every morning.

Evidence for the use of light to treat SAD has been around for more than a decade, yet doctors were slow to adopt the practice, says Raymond Lam, a psychiatrist at the University of British Columbia in Vancouver. “One reason is that doctors are used to prescribing medicine,” he says.

In 2006, Lam embarked on a study to compare light therapy with the anti­depressant Prozac in patients with severe SAD; he found that the treatments were equally good. If anything, the light therapy worked sooner — within a week — and caused fewer side effects, but over the eight-week trial, two-thirds of people in each group responded favorably to treatment.

A recent study found that even one episode of light treatment improved mood in SAD patients — at least on that day.

With three approaches to choose from — light, medicine and talk therapy — patient preference comes into play. For instance, light therapy takes time — a half-hour a day — and the cost of buying a light ($100 or more) may not be covered by insurance.

Some people use more than one approach. “In my clinical experience, many patients feel better with a combination of treatments,” says Teodor Postolache, a psychiatrist at the University of Maryland School of Medicine. Some address the problem by traveling south for the winter.

Why does light work? Several strands of evidence suggest that a deficit in light is the main trigger for SAD. The disorder is more common at higher latitudes, where winter daylight is limited. The timing of melatonin, a hormone involved in the sleep-wake cycle, is altered by the change of seasons and sometimes goes awry, Postolache says, and as a result, “SAD patients tend to extend their biological night.”

When light hits our eyes, it triggers neural activity that affects circadian rhythms, says Kathryn Roecklein, a psychologist at the University of Pittsburgh. So morning light exposure may help with the shift from sleep to wakefulness, from biological night to day.

Roecklein has found a biological difference between people with and without SAD. In healthy people, the eye’s sensitivity to light — measured by the pupillary response and presumably related to light’s ability to affect the biological clock — increases in wintertime. However, in people with SAD, the sensitivity to light is diminished.

While SAD may occur in 1 to 2 percent of the population, Lam says, milder versions of the ailment — the winter blahs — affect 10 to 15 percent of people. A mild case is more likely than full-blown SAD to be accompanied by physiological symptoms — oversleeping, overeating, carbohydrate craving and low energy — but less likely to prompt psychological symptoms such as intensely low mood.

“It looks like light therapy helps with milder forms of SAD,” Lam says, although he says this has not been nearly as well studied as more severe SAD.

Just because “happy lights” are readily available doesn’t mean you should experiment. There are reasons for caution.

One concern is self-diagnosis. Postolache says there are other wintertime factors that can affect health, such as infectious illness and indoor allergies. “There are phenomena that aren’t SAD, but seasonal moods” that might relate to the anniversary of a loss or unrealized expectations of joy and celebration around the holidays, he says.

A more serious concern is the risks that accompany untreated or undertreated depression. “If your symptoms are severe enough to interfere with daily functioning, get an assessment at a mental health clinic or by a doctor,” Lam says. “There are many causes and lots of treatments.”

Also, be careful if you’re taking medication (such as lithium) or have a condition (such as retinal disease) in which photosensitivity is an issue. “Otherwise, it’s a pretty safe treatment,” Lam says.

Roecklein advises people who think they may have SAD to consult a physician or psychologist with experience using light therapy. “A doctor can tailor the duration and timing of the light to the patient’s needs,” she says.

Correction: An earlier version incorrectly identified Kathryn Roecklein as Karen Roecklein. This version has been corrected.