The man who eats too little, or the man who eats too much? The woman who can't sleep, or the woman who can't stay awake? The sad sack that doesn't react to what others say or do, or the flighty one who overreacts to everything? The man who was fine until his mid-30s, or the man who's suffered since childhood?

Each of these people shows signs of depression. That may seem odd because we've come to think of depression as one thing. It's not.

And pretending that it is one thing is bad for patients. It's bad for therapists. And it's bad for clinical trials of antidepressant drugs, says Jonathan W. Stewart, MD, professor of clinical psychiatry at Columbia University and research psychiatrist at New York State Psychiatric Institute.

"The fact is, we don't know what the different illnesses are, all of which we call depression," Stewart tells WebMD.

Depression Means Different Things to Different People

Depression turns out to be a very general term.

"You come into the doctor's office and you say, 'I think I've got a fever.' He takes your temperature and says, 'By golly, you do have a fever.' It's the same with depression," Stewart says. "Someone comes to my office and tells me he feels depressed. I ask him to tell me about it and then I say, 'Yes, you have major depression.' It is like saying, 'You've got a fever all right.' We don't know -- just as the doctor with the fever patient doesn't know -- the actual cause of the problem."

That's a startling admission, given that Stewart is widely considered an expert in the diagnosis and medical treatment of depression.

But other experts say the same thing. One is David D. Burns, MD, clinical professor of psychiatry and behavioral sciences at the Stanford University School of Medicine and visiting scholar at Harvard Medical School. Burns' best-selling book, Feeling Good: The New Mood Therapy, and the more recent Feeling Good Handbook are the books most often recommended to depressed patients by psychologists and psychiatrists.

Burns notes that there's recently been a lot of interest in defining the different types of depression, particularly a diagnosis called "atypical depression." The key symptoms of atypical depression are overeating, oversleeping, fatigue, extreme sensitivity to rejection, and moods that worsen or improve in direct response to events. (Regular depression tends to be marked by pervasive sadness.)

The increased interest in atypical depression is part of an effort, Burns says, to link different aspects of depression to specific brain disorders. Perhaps one day that effort will bear fruit, but Burns isn't optimistic about that.

"I am worried that our field is moving in the wrong direction," he tells WebMD.