"During the night, he underwent respiratory assistance," which helped stabilise his condition after he was rushed to hospital with breathing difficulties, spokesman Joaquin Navarro-Valls said outside the hospital on Wednesday.
He gave no details of what such assistance entailed.
The 84-year-old pope slept "for a few hours" during the night, and had "very little fever", he added, in the first medical bulletin on his condition in more than 12 hours.
"When I left him, the pope was preparing to celebrate mass with his secretary [Monsignor Staneslaw Zewesz]," the spokesman said.
"When I left him, the pope was preparing to celebrate mass with his secretary [Monsignor Staneslaw Zewesz]"
He denied suggestions that the pontiff had to undergo a tracheotomy to help him breathe, and added that he did not undergo a CAT scan, as reported by Italian media.
The leader of the Roman Catholic Church, who is also afflicted by Parkinson's disease, was rushed to hospital late on Tuesday with breathing difficulties after a bout of influenza suddenly worsened.
The Vatican said he had suffered an acute attack of laryngospasm, a blockage of air to the lungs, and needed urgent medical attention.
"The flu condition that has afflicted the Holy Father for the past three days deteriorated tonight with an acute laryngospasm. For this reason, it was decided to urgently take the pope to the Gemelli hospital," a statement had said.
The Polish pontiff has been treated at least six times in the Gemelli during his 26-year papacy, and has his own suite on the 10th floor of the hospital on the outskirts of Rome.
Medical experts said acute laryngospasm was very rarely fatal, but cautioned that his age and medical history complicated matters.
"He's an elderly man and that's a serious problem for an elderly patient," said Gerald Berke, chief of head and neck surgery at the University of California at Los Angeles Medical Centre.
"He's an elderly man and that's [laryngospasm] a serious problem for an elderly patient"
Los Angeles Medical Centre
The pope has occasionally had difficultly breathing since he came down a decade ago with Parkinson's, a disorder of the central nervous system which can affect chest muscle activity.
The flu apparently complicated his condition. He cancelled all his public engagements for Monday, Tuesday and Wednesday. The last time he skipped an audience for health reasons was in September 2003, when he had an intestinal ailment.
The sudden worsening of his health again raised fears that one of the most remarkable papacies might be drawing to a close.
John Paul II has put his his mark on the world like few others in the 20th century, credited with a leading role in the fall of communism in Eastern Europe in 1989, and also working tirelessly to revolutionise relations with other religions.
While nearly all of the world's one billion Catholics have cheered his defence of human rights, he has drawn criticism from within his own flock for his unbending stance against contraception, gay marriages and women priests.
A handful of well-wishers, defying the winter chill, gathered outside the gates of the Gemelli hospital early on Wednesday morning as word of the pope's illness spread.
Rome has been hit by an unusual cold snap and the winter chill has coincided with an outbreak of influenza across Italy, that has laid up one out of 100 people.