CBS News correspondent Allen Pizzey reports that a Vatican spokesman said the decision to move the mass forward was taken nearly two months ago, "to tire the pope a little less."
The pontiff has "a very full schedule for a person more than 80 years old," the spokesman added.
The mass generally lasts about 90 minutes, and the Pope has to be back in St Peter's Square on Christmas morning to deliver the "Urbi et Orbi," his annual message to the city and the world.
Some observers have noted that Pope John Paul II kept to the traditional schedule even in 2004 when his various illnesses were well advanced, and visually obvious.
Benedict suffered a mild stroke in 1991, is thought to have had another one just before he was elected Pope, and in a fall while on holiday in the Italian Alps in July.
Vatican sources, who will only speak off the record, say Benedict is "tired," and meets with only a small number of close advisers.
His personal secretary, Monsignor Georg Ganswein, is said by Vatican insiders to be assuming more and closer control over Benedict's schedule.
Benedict will make four foreign trips and several inside Italy in the coming year, with requests for more, but concerns for his health mean his staff - not his flock - will have the biggest influence on the Pope's calendar.