Father Federico Lombardi said that the decision to hold the mass at ten in the evening had been taken nearly two months ago.
The service would end at midnight rather than starting at midnight in order to "to tire the Pope a bit less" and enable him to retire to bed earlier to rest before before the rigours of Christmas Day, when he reads his "Urbi et Orbi" message to the city and the world.
"There is no cause for alarm," Father Lombardi said.
Andrea Tornielli, the biographer of Pope Benedict and other modern Popes, said however that Pope John Paul II had never varied the Christmas liturgical calendar and had always held the mass at midnight, even in the final years of his decline.
He died in 2005.
The German-born pontiff, 82, is committed to a busy travel schedule for 2010, including a planned trip to Britain in the autumn during which he will beatify Cardinal John Henry Newman, the celebrated nineteenth century Anglican convert to Roman Catholicism.
The UK visit also comes against the background of Pope Benedict's opening to Anglicans who wish to convert to Rome while retaining their Anglican traditions and practices, seen by some as part of his drive to re-unite the Christian world but by others as a divisive move.
In April the Pope is to visit Malta. In May he will go to Turin to inaugurate a public display of the Holy Shroud of Turin, said to be the burial cloth of Jesus, and the Marian shrine at Fatima in Portugal. In June he is due to visit Cyprus.
Pope Benedict, the former Cardinal Joseph Ratzinger, is known to have repeatedly asked to retire as Head of the Doctrine of the Faith and Pope John Paul's right hand man and devote himself to scholarship.
However Pope John Paul refused to let him resign, and four years ago the College of Cardinals elected him as John Paul's successor.
Pope Benedict suffered a mild stroke in 1991, and is believed to have suffered another not long before his election as Pope. He has twice fallen while on holiday in the Italian Alps, the first time in August 1992 and the second time in July this year.
On the first occasion he hit his head on a radiator, and on the second he broke his wrist.
The Vatican denied that he had fainted, saying he had slipped during the night.
He has however appeared in good form recently during public appearances, with no sign of fatigue.
On Sunday he tackled the issue of climate change in his Angelus address on St Peter's Square, calling for "responsible" action on the environment to give relief to "the poor and future generations" ahead of the UN climate conference in Copenhagen.
"Protecting the natural world calls for restrained and responsible lifestyles, especially in consideration of the poor and future generations," the Pope said.
"I call on all people of good will to respect the laws of God on nature and to rediscover the moral dimension in human life".
The Vatican will be represented at Copenhagen by Monsignor Celestino Migliore, the Vatican's permanent observer at the United Nations.
The Pope also said that human history was "moved by the Word of God". He noted that the Gospel text of the day, from St Luke, had "an abundance of references to all the political and religious authorities of Palestine in 27-28 AD. Evidently the evangelist wants to point out to the reader or listener that the Gospel is not a myth, but the account of a true story, that Jesus of Nazareth is a historical personage in a precise context."
He quoted St Ambrose as saying, "So, the Word descended that the Earth, which before had been a desert, would produce its fruits for us."
The Pope added however that "in the Church there is always a struggle taking place between the desert and the garden, between the sin that parches the earth and the grace that waters it so that it produces abundant fruits of holiness ... Let us therefore pray to the Mother of the Lord that she will help us, in this Advent season, to straighten our ways, letting ourselves be guided by the word of God."
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