The cause may differ, as well. Seasonal depression in the winter seems linked to increases in the production of melatonin, a chemical that helps set the brain's daily rhythm, set off by the decrease in light.

But ''the seasonal trigger for the summer depression is less clear-cut,'' said Dr. Norman E. Rosenthal, a Washington psychiatrist and the author of ''Winter Blues.'' ''Conventionally, the thought has been that they are more sensitive to the heat. The question of whether it's too much heat or too much light has yet to be resolved.''

Reports of summer seasonal affective disorder are often more frequent in hotter regions. A study published in the journal Comprehensive Psychiatry two years ago found that the rate of summer SAD among a group of students in Jining, 200 miles northwest of Beijing, exceeded that of students with the winter disorder. Epidemiological data in the United States have shown a higher proportion of people in the South depressed in the summer. The proportion rises as the latitude diminishes.

When moods deviate, Dr. Rosenthal said, the systems geared toward normalizing them generally take action. In seasonal affective disorder, he said, ''The challenges encountered with changing seasons seem to overwhelm those internal regulating mechanisms.''

Dr. Rosenthal and Dr. Wehr first identified winter SAD in 1984. Their findings prompted queries from many people who said they also felt depression, but in the summer.

To explore the summer disorder, Dr. Wehr manipulated patients' body temperatures. People with severe depression, he said, tend to have higher temperatures at night; among healthy people, temperatures tend to drop. Antidepressants have been shown to lower brain and body temperature.

Dr. Wehr tried to cool down patients with a kind of reverse thermal blanket, carefully making sure the environmental drop in temperature would not cause shivering as a defense against the cold. After the treatment was over, however, the patients walked out of the building into summer heat, their body temperatures rose, and the symptoms of their depression returned. The effect of re-entering a hot summer environment undid whatever effect the treatment might have had.