Senate Majority Leader Harry Reid (D-Nev.) went to the hospital Friday morning after not feeling well, his office said.
"Early this morning, Senator Reid was not feeling well and as a precaution decided to go to the hospital. Tests have been conducted and everything is normal. He is alert, resting and feeling better," said spokesman Adam Jentleson in a statement. "Doctors have asked that he remain in the hospital for observation so he will not be working today."
Reid's hospitalization came after a late night of work in the Senate Thursday, with the chamber striking a deal near midnight to avoid an all-night session.
The Senate is in its last day of business Friday until next year. Six votes on nominees are scheduled for the day, including a motion to invoke cloture on the nomination of Federal Reserve Chair Janet Yellen.
The 74-year-old senator was hospitalized briefly in October 2012 after his caravan was in a car crash in Las Vegas. He only suffered minor injuries in the incident.