Many of us can hardly wait for summer to arrive, but a small number of people are much happier when it's over. You've no doubt heard of SeasonalAffectiveDisorder, the wintertime mood disorder -- but some get SAD in the summer.

As hot weather approaches, those with summer SAD sleep less, eat less, and lose weight. They're extremely irritable and agitated. (It's the reverse for people with winter SAD, who sleep more, gain weight and crave high-carb foods, and tend to slow down and socially hibernate from late fall to early spring.)

Summer-onset depression is thought to affect less than 1 percent of the population, making it much rarer than the winter variety experienced by an estimated 5 percent of people.

In its most severe form, people with summer seasonal depression may be more at risk for suicide than cold-weather SAD, says Dr. Norman Rosenthal, a clinical professor of psychiatry at Georgetown University Medical School, who has studied both types and first helped discover their existence. "Suicide is more of a concern when people are depressed and agitated rather than depressed and lethargic," he explains.

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When summer depression was first recognized in 1986, Rosenthal said that mental health professionals suspected the cause was the heat and humidity. That, he said, lent itself to the idea that a cold shower, air conditioning, swimming in cold lakes or heading North would relieve symptoms. Although these treatments for hot-weather depression are useful for some, they lack the staying power that light-box therapy has on winter SAD.

'The light is cutting though me like a knife'

A person with summer SAD can stay inside, crank up the AC, and darken the room but then go outside into the heat and it's as if they've never been treated, explains Rosenthal, the author of "Winter Blues."

Another idea is that it might be the light itself that's aggravating sufferers, whether it's the intensity of sunlight or the angle it's coming at people. One of Rosenthal's summer depression patients describes it as "feeling like the light is cutting though me like a knife."

Still another possibility is that there may be two kinds of warm-weather depression, says Dr. Alfred Lewy, a professor of psychiatry at Oregon Health & Science University in Portland. He suggests there might be one group of people who have an unpleasant reaction to the heat and humidity -- a discomfort with the climate. But even in Portland where summers aren't that hot or humid, he's seen patients struggle with summer depression.

Lewy suspects the cause in a second group might be that the body's natural clock, it's circadian rhythms, are misaligning in summer. Instead of cueing to dawn, the longer daylight is causing some vulnerable people to cue to dusk. Cueing to dusk shortens the typical body clock and delays a person's sleep-wake cycle. This mismatch, theorizes Lewy, may be triggering depression.

He successfully treated a person with summer depression with a combination of getting early morning sunlight (30 to 60 minutes daily), which shifts the body clock forward, and low-dose melatonin, a hormone that helps regulate sleep-wake cycles. Severe symptoms may also benefit from antidepressants.

Do you secretly -- or perhaps not-so-secretly -- loathe the summer months? What helps you get through them?