Jackson's office announced last week he had been on medical leave more for than two weeks because of exhaustion.
A spokesman for Jackson, Frank Watkins, said Jackson was being treated at an undisclosed "in-patient medical facility" and his doctors have said he "will need to receive extended in-patient treatment, as well as continuing medical treatment thereafter."
"Recently, we have been made aware that he has grappled with certain physical and emotional ailments privately for a long period of time," Watkins said.
The statement provided no further details about the condition or how long Jackson's medical leave would last, WBBM-AM, Chicago, reported.
Raghuveer Nayak, a friend of Jackson, was arrested on federal fraud charges recently. The charges involved Nayak's surgical centers. Nayak had been at the center of the U.S. Senate seat scandal that led to former Gov. Rod Blagojevich being sent to prison.
Jackson's chief of staff, Rick Bryant, has said Nayak's arrest on racketeering, fraud and tax evasion charges was not connected to Jackson's leave of absence.
The Chicago Tribune has reported the House Ethics Committee is investigating allegations Nayak offered Blagojevich as much as $6 million in campaign funds to make Jackson President Barack Obama's successor in the U.S. Senate.
Jackson had denied he knew of fundraising in exchange for the appointment.