The congressman's exact diagnosis isn't known, but a statement attributed to his physician says he's being treated for a mood disorder.
Classifying something as a mood disorder may bring more questions than answers because the term mood disorder is a broad umbrella category of mental illness.
According to experts it can include: minor to major depression, bipolar disorder, seasonal affective disorder and mood disorders associated with alcohol and drug abuse.
Depression is more common than other mood disorders and can affect up to 5 percent of men at some point.
Symptoms can range from trouble sleeping, to hopelessness, to thoughts of suicide.
University of Illinois-Chicago psychiatrist Luan Phan says these disorders can be quite serious and may involve more than one ailment.
"It's disabling that it interferes with your ability to function with your job, with your interpersonal relationships at home work or among all spheres." Says Phan, "It is telling that [this] has lead to month-long hospitalization ... It really speaks to the complexity of the condition."
The congressman's exact ailment is not known but Jackson's wife, Alderman Sandi Jackson, says her husband is not being treated for alcohol or substance abuse.