Jackson was not in Washington last week and his last recorded vote was June 8, Roll Call reported. A statement from his office said his leave started June 10.
The statement said the 47-year-old congressman is "on a medical leave of absence and is being treated for exhaustion."
"He asks that you respect his family's privacy," the statement said. "His offices remain open to serve residents of the Second District."
Frank Watkins, a spokesman for Jackson's office in Washington, told the Chicago Tribune he knew where the congressman was being treated, but declined to say where at the family's request.
Watkins said he didn't know how long Jackson would be away from his job and said he did not need permission from House officials to take a leave, the Tribune said.
Jackson is the subject of a House Ethics Committee probe into allegations a longtime friend, Raghuveer Nayak, offered then-Illinois Gov. Rod Blagojevich as much as $6 million in campaign donations to appoint Jackson to the U.S. Senate seat vacated by President Barack Obama. Blagojevich was convicted and sent to prison for his efforts to parlay the Senate vacancy into a financial or political windfall.
Jackson has denied any involvement in efforts to swing the Senate seat his way.
Navak was arrested on federal fraud charges involving his surgical centers.