Pope Francis's medical condition is "critical" as respiratory and kidney problems, the Vatican said.
"The night went well, the Pope slept and is resting," the Vatican said on Monday.
Vatican sources said the Pope was in good humour and eating normally following his breathing difficulties on Saturday.
The Pope was admitted to Rome's Gemelli Hospital last Friday after experiencing breathing difficulties for several days, where he was first treated for bronchitis before being diagnosed with pneumonia in both lungs.
On Sunday, the Pope's thrombocytopenia was stable, according to the Vatican.
People in Rome are gathering outside St Peter's Basilica to lead prayers for the Pope, together with members of the Vatican curia and clergy from the Diocese of Rome.
Monday evening's prayer is expected be led by Cardinal Parolin, who is Vatican's secretary of state.