SAD is normally associated with depression, lethargy, fatigue and other symptoms that coincide with the colder, darker months of the year. According to the National Institute of Mental Health, up to 10 million people in the United States have some degree of winter-onset depression.
A number of studies dating back to the early 1980s suggest that these depressive symptoms are consequences of too little light, and today researchers believe that a dearth of light can lead to low levels of serotonin in the brain. This chemical plays an important role in mood regulation and proper sleep-wake cycles.
Dr. Alfred Lewy, director of the Sleep and Mood Disorders Laboratory at Oregon Health and Science University, published a paper in the journal Proceedings of the National Academy of Sciences three years ago in which he and his colleagues found convincing evidence that winter SAD was a result of a concept known as "circadian misalignment" -- a jet lag-like disruption of sleep/wake patterns brought about by a change in the seasons.
The root cause of summer depression is much less clear. Lewy said that the first real studies conducted to determine the existence of a summer version of SAD were conducted in 1991 by Dr. Thomas Wehr of the National Institute of Mental Health and colleagues. He said that these initial studies showed that in general, sufferers tended to experience a different range of symptoms than their winter SAD counterparts.
"In people with summer depression, you see a decreased appetite and insomnia; with winter depression, you get an increased appetite and increased sleep," Lewy said.
It is a feeling with which Smith is familiar.
"I started getting anxiety... It's very difficult because there is nowhere to escape," Smith said. "I liken it to feeling like a little insect under a magnifying glass. All you can do is hide away in a dark place."
But because cases of summer depression are relatively rare -- less than 1 percent of the population, by some estimates -- researchers in the field have a difficult time finding enough people with the condition to conduct large studies. Lewy said he has encountered only a few patients who seem to have depression linked to the approach of summer.
Still, he said, this does not mean that summer-onset depression doesn't exist.
"It's a fairly rare disorder, but I think Dr. Wehr is on to something," Lewy said. But as for the underlying causes, he added, "We don't know the answer to that."
Smith said that when she has too much exposure to the bright sun of summer, her symptoms can range from irritability to anxiety and panic attacks.
"It becomes very disabling at a certain point," she said. "At my lowest point I felt very suicidal."
Fortunately, Lewy said, existing treatments for depression are generally effective for those who experience similar symptoms. "As far as I know, the antidepressant that is used to treat non-seasonal depression has also been used in treating summer depression and winter depression," he said.