Researchers say that some people may be especially sensitive to heat, which could influence their production of various hormones. "We know thyroid hormone is suppressed by heat and growth hormone and prolactin are stimulated by heat," Wehr says.
Lack of thyroid hormone can cause energy drain, and too much growth hormone and prolactin can lead to lethargy and lack of libido. In addition, prolactin is known to repress the effects of dopamine, a brain chemical linked to feelings of enjoyment and pleasure. Light on the skin can also influence the production of hormones, but preliminary NIMH research points to heat more than light as the culprit in summer depression.
Typical to depression in general, researchers say, more women than men appear to suffer from summer SAD. The condition generally crops up in childbearing years and studies point to a genetic link; more than two-thirds of SAD patients have a first-degree relative with a mood disorder.
"The problem is that right now we just don't know enough about summer depression to say it gives us prognosis and treatment," says Dr. Daniel F. Kripke, professor of psychiatry at UC San Diego. "Until we do, it's not very useful."
Winter blues responds well to light therapy, and because researchers think the summer version is a condition of heat, reducing heat should work as a treatment for summer depressives.
That doesn't seem to be the case. A pilot study by Wehr and colleagues at NIMH found that manipulating temperature for a period of time was salutary but didn't have lasting effects on summer SAD patients.
"In our study, people would improve [with cooling] and then very quickly the symptoms would come back," Wehr says.
Antidepressants help
Some researchers take issue with associating winter depression with summer depression or labeling it "reverse SAD."
"Calling it 'reverse SAD' only confuses the issue," says Michael Terman, director of the Center for Light Treatment and Biological Rhythms at New York-Presbyterian Hospital. "It is not a light-related phenomenon and should not be considered the flip side of winter depression." Standard antidepressants are the only medical intervention that's been shown to be effective, he says.
Wehr agrees medication tends to work, but says the drugs may vary. "Some respond to Prozac-like drugs, some people respond to Wellbutrin. I've even had people respond to lithium and only lithium. They only take it during the summer," Wehr says.
