Pope Francis, who has been struggling to overcome double pneumonia for almost three weeks, had a restful night in hospital, the Vatican said on Wednesday.

The 88-year-old pope was admitted to Rome's Gemelli hospital on February 14 with a severe respiratory infection that has required continuously evolving treatment.

The Vatican said on Tuesday that Francis would be reconnected to non-invasive mechanical ventilation overnight to help him breathe, underscoring his continued difficulty in shaking off the dangerous illness.

"The pope rested well during the night," the Vatican said on Wednesday, adding only that he had woken up shortly after 8:00 a.m. (0700 GMT). The pontiff normally starts his day well before dawn when he is healthy.

Francis suffered what the Vatican described as two episodes of "acute respiratory insufficiency" on Monday. Tuesday's health bulletin was more upbeat, saying the pope was stable throughout the day and did not have any new breathing crises.

However, his doctors reiterated again that his prognosis was "guarded," meaning he was not yet out of danger.

The pope has not been seen in public since entering hospital, his longest such absence since his papacy started 12 years ago. His doctors have not said how long his treatment might last.