Although the mechanism of the disorder is not perfectly understood, it is known to be a reaction to the biological effects of light on the body (see biorhythm). Daily, or circadian, rhythms help animals keep track of the seasonal changes in the environment, such as the shortening of days in winter, so that they can make the adaptive changes necessary for their survival in each season. Two substances, the hormone melatonin and the neurotransmitter serotonin, are a part of this process and are being studied for a possible role in SAD. Melatonin is secreted by the pineal gland, which is in turn controlled by an area (the suprachiasmatic nuclei) of the hypothalamus; the hypothalamus, among other things, performs a clocklike function in the body. The eye's retinal nerves are connected to this area. Melatonin is secreted chiefly at night, and its secretion is suppressed by light. Secretion of the neurotransmitter serotonin declines in the winter and may undergo abnormal declines in those with SAD; concentrations of serotonin are increased by bright light. Serotonin is especially active in the hypothalamus. Decreased sensitivity of the retina has also been implicated as a cause of SAD.
Treatment with bright light (about five to twenty times brighter than normal lighting) often alleviates symptoms within a period of days. Unwieldy lighting paraphernalia has given way to smaller, portable light boxes and lighted visors. Doses range from 30 minutes to a few hours per day, often undergone in the morning to simulate the dawn.
There are many different treatments for classic (winter-based) seasonal affective disorder, including light therapies with bright lights, anti-depression medication, ionized-air reception, cognitive-behavioral therapy and carefully-timed supplementation of the hormone melatonin.
SAD can be a serious disorder and may require hospitalization. There is also potential risk of suicide in some patients experiencing SAD. One study reports 6-35% of sufferers required hospitalization during one period of illness. The symptoms of SAD mimic those of dysthymia or clinical depression. At times, patients may not feel depressed, but rather lack energy to perform everyday activities. Norman Rosenthal, a pioneer in SAD research, has estimated that the prevalence of SAD in the adult United States population is between about 1.5 percent in Florida and about 9 percent in the northern US.
Various etiologies have been suggested. One possibility is that SAD is related to a lack of serotonin, and serotonin polymorphisms could play a role in SAD, although this has been disputed. Mice incapable of turning serotonin into N-acetylserotonin (by Serotonin N-acetyltransferase) appear to express "depression-like" behaviors, and antidepressants such as fluoxetine increase the amount of the enzyme Serotonin N-acetyltransferase, resulting in an antidepressant-like effect. Another theory is that the cause may be related to melatonin which is produced in dim light and darkness by the pineal gland, since there are direct connections, via the retinohypothalamic tract and the suprachiasmatic nucleus, between the retina and the pineal gland.
Subsyndromal Seasonal Affective Disorder is a milder form of SAD experienced by an estimated 14.3% vs. 6.1% of the U.S. population. The blue feeling experienced by both SAD and SSAD sufferers can usually be dampened or extinguished by exercise and increased outdoor activity, particularly on sunny days, resulting in increased solar exposure. Connections between human mood, as well as energy levels, and the seasons are well-documented, even in healthy individuals.
The Mayo Clinic describes three types of Seasonal Affective Disorder, each with its own set of symptoms. According to the American Psychiatric Association, for a diagnosis to qualify as SAD, it must meet four criteria: depressive episodes at a particular time of the year; remissions or mania/hypomania also at a characteristic time of year; these patterns must have lasted two years with no nonseasonal major depressive episodes during that same period; and these seasonal depressive episodes outnumber other depressive episodes throughout the patient's lifetime.
There are many different treatments for classic (winter-based) seasonal affective disorder, including light therapies, medication, ionized-air reception, cognitive-behavioral therapy and carefully timed supplementation of the hormone melatonin.
Bright light treatment is common although up to 69% of patients find it inconvenient and as many as 19% of patients stop use because of this. A specially designed lamp, many times brighter than normal indoor lighting, is placed near the sufferer, and has proven to be effective at doses of 2500 - 10,000 lux. Most treatments use 30-60 minute treatments, however this varies depending on the situation. The sufferer sits a prescribed distance, commonly 30-60 cm, in front of the box with her/his eyes open but not staring at the light source. Many patients use the light box in the morning, however it has not been proven any more effective than any other time of day. Discovering the best schedule is essential. Dawn simulation has also proven to be effective; in some studies, there is an 83% better response when compared to other bright light therapy. When compared in a study to negative air ionization, bright light was proven to be 57.1% effective vs. dawn simulation, 49.5%. Patients using light therapy can experience improvement during the first week, but increased results are evident when continued throughout several weeks. Most studies found it effective without use year round, but rather as a seasonal treatment lasting for several weeks until frequent light exposure is naturally obtained.
SSRI (selective serotonin reuptake inhibitor) antidepressants have proven effective in treating SAD. Bupropion is also effective as a prophylactic. Effective antidepressants are fluoxetine, sertraline, or paroxetine. Both fluoxetine and light therapy are 67% effective in treating SAD according to direct head-to-head trials conducted during the 2006 CAN-SAD study. Subjects using the light therapy protocol showed earlier clinical improvement, generally within one week of beginning the clinical treatment.
Negative air ionization, involving the release of charged particles into the sleep environment, has also been found effective with a 47.9% improvement. Depending upon the patient, one treatment (ie. lightbox) may be used in conjunction with another therapy (ie. medication). Modafinil may be also an effective and well-tolerated treatment in patients with seasonal affective disorder/winter depression.
Alfred J. Lewy of Oregon Health and Science University in Portland, OHSU, and others see the cause of SAD as a misalignment of the sleep-wake phase contra the period of the body clock, circadian rhythms out of synch, and treat it with melatonin in the afternoon. Correctly timed melatonin administration shifts the rhythms of several hormones en bloc.
Around 20% of Irish people are affected by SAD, according to a survey conducted in 2007. The survey also shows women are more likely to be affected by SAD than men. An estimated 10% of the population in the Netherlands suffers from SAD.