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Seasonal affective disorder, or SAD, also known as winter depression is an affective, or mood disorder. Most SAD sufferers experience normal mental health throughout most of the year, but experience depressive symptoms in the winter. SAD is rare, if existent at all in the tropics, but is measurably present at latitudes of 30°N (or S) and higher.
Connections between human mood, as well as energy levels, and the seasons are well-documented, even in healthy individuals. Particularly in high latitudes (50°N or S) it is common for people to experience lower energy levels during the winter. Colds and flu also peak during this time, and most people get less outdoor exercise than in the summer.
Norman Rosenthal, MD, pioneer in SAD research, has estimated that the prevalence of SAD in the adult US population is between 1.4 percent (in Florida) and 9.7 percent (in New Hampshire).
Seasonal mood variations are believed to be related mostly to daylight, not temperature. For this reason, SAD is prevalent even in mid-latitude places with mild winters, such as Seattle. People that live in the Arctic region are especially susceptible due to the effects of polar nights. Prolonged periods of overcast weather can also exacerbate SAD. Normal "winter blues" can usually be dampened or extinguished by exercise and increased outdoor activity, particularly on sunny days, resulting in increased solar exposure. SAD, however, is a more serious disorder, sometimes triggering dysthymia or clinical depression. It may require hospitalization.
Various etiologies have been suggested. One possibility is that SAD is related to a lack of serotonin and that exposure to full-spectrum artificial light may improve the condition by stimulating serotonin production, although this has been disputed., Another theory is that melatonin produced in the pineal gland is the primary cause,, since there are direct connections between the retina and the pineal gland. Some studies show that melatonin levels do not appear to differ between those with and without SAD. However, mice incapable of synthesizing melatonin appear to express "depression-like" behaviors, and melatonin receptor ligands produce antidepressant-like effect in mice. Light therapy appears to be effective in treating SAD, but the exact mechanism of the effect is still unknown.
Full-spectrum bulbs and "light boxes" can be purchased as specialty lighting products for those suffering from SAD. These light boxes are many times brighter than regular indoor lighting and should be administered with guidance from a psychiatrist. Natural sunlight is considered to be the best source of light. 
The most validated of the light therapies is the use of a bright light box at a dose of 10,000 lux for 30-60 minutes daily. The eyes should remain open during this time, but not directly staring at the lights. The best time to administer the therapy is currently in debate. However, some find that daily morning therapies work well. The antidepressant effect of light therapy, like antidepressant medications, may take several weeks to take full effect. Some improvement should be noted within a week. Light therapy should be continued until natural daily light exposure becomes sufficient, usually during spring.
Medication is a more recent treatment and selective serotonin reuptake inhibitors (SSRIs) have proven effective in treating SAD. Examples of these antidepressants are fluoxetine (Prozac), sertraline (Zoloft, Lustral), or paroxetine (Paxil). 
Several controlled studies have shown dawn simulation to be as effective as bright lights in treating SAD, with fewer side effects and greater convenience. A specialized control device called a dawn simulator gradually brightens ordinary bedside lights during the hour before the patient awakens. 
Winter depression (or winter blues) is a common slump in the mood of Scandinavians. Doctors estimate that about 20% of all Swedes are affected, and it seems to be hereditary. It was first described by the 6th century Goth scholar Jordanes in his Getica where he described the inhabitants of Scandza (Scandinavia). In the USA the diagnosis of seasonal affective disorder was first proposed by Norman E. Rosenthal, MD in 1984. Rosenthal wondered why he became sluggish during the winter after moving from sunny South Africa to New York. He started experimenting increasing exposure to artificial light, and found this made a difference.
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- ^ Seasonal affective disorder: autumn onset, winter gloom - board review
- ^ Johansson C, et al. (2001) Seasonal affective disorder and serotonin-related polymorphisms. Neurobiol Dis. 8(2):351–7.
- ^ Johansson C, et al. (2003) The serotonin transporter promoter repeat length polymorphism, seasonal affective disorder and seasonality. Psychol Med. 33(5):785–92.
- ^ The Merck manual
- ^ National mental health association article
- ^ Prolonged swim-test immobility of serotonin N-acetyltransferase (AANAT)-mutant mice
- ^ Reliability assessment of an automated forced swim test device using two mouse strains
- ^ The antidepressant-like effect of the melatonin receptor ligand luzindole in mice during forced swimming requires expression of MT2 but not MT1 melatonin receptors
- ^ UBC FAQ for SAD
- ^ SAD information from the WSC Counseling Center
- ^ Avery DH et al. (2001) Dawn simulation and bright light in the treatment of SAD: a controlled study. Biol Psychiatry. 50(3):205–16.
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