DSM IV criteria for Seasonal Affective Disorder (SAD).
Any depressive disorder, be it recurrent depressive episode, major depressive episodes in bipolar I disorder, bipolar II disorder can have a seasonal pattern in which case the specification seasonal pattern can be applied if the following criteria apply.
a. There has been a regular temporal relationship between the onset of major depressive episodes and a particular time of year (e.g. regular appearance of the major depressive episode in autumn or winter)
b. Full remissions (or a change from depression to mania or hypomania) also occur at a characteristic time of year (e.g. depression disappears in the spring).
c. In the last 2 years, two major depressive episodes have occurred that demonstrate the temporal season relationship defined in criteria a and b, and no non-seasonal major depressive episode has occurred during the same period.
d. Seasonal major depressive episodes (as described above) substantially outnumber the non-seasonal major depressive episodes that may have occurred over the individual’s lifetime.
Note: Do not include cases in which there is an obvious effect of season-related psychosocial stressors (e.g. regularly being unemployed each winter)
Unlike classically depressed patients, most SAD patients develop ‘atypical’ symptoms of increased fatigue, increased sleep duration and increased appetite and weight. Not only do SAD patients crave carbohydrates, but also they actually report eating more carbohydrate-rich foods in the winter.
Especially these atypical symptoms and the disappearance of symptoms after the winter makes it hard to diagnose this variant of depression.
How is SAD treated with light therapy?
An excellent recent post about SAD explains treatment of SAD with light therapy: More Than Coping: Seasonal Affective Disorder
Bright white fluorescent light has been shown to reverse the winter depressive symptoms of SAD. Early studies used expensive “full-spectrum” bulbs, but these are not especially advantageous. Bulbs with color temperatures between 3000 and 6500 degrees Kelvin all have been shown to be effective. The lower color temperatures produce “softer” white light with less visual glare, while the higher color temperatures produce a “colder” skylight hue. The lamps are encased in a box with a diffusing lens, which also filters out ultraviolet radiation. The box sits on a tabletop, preferably on a stand that raises it to eye level and above. Such an arrangement further reduces glare sensations at high intensity, and preferentially illuminates the lower half of the retina, which is rich in photoreceptors that are thought to mediate the antidepressant response. Studies show between 50% and 80% of users showing essentially complete remission of symptoms, although the treatment needs to continue throughout the difficult season in order to maintain this benefit.
A lot is written and has been written about SAD in blog posts, treatment, causes, symptoms, you can find links to 8 Blogs in this article on this blog: 8 articles about Seasonal Affective Disorder