Christmas is a heady brew of family, religion, myth, spectacle, presents, cooking, stress and the supernatural. We cast a scientific eye over what can often be a bit of an emotional rollercoaster. Gift giving: Present giving is an ancient behaviour that predates Christmas – which may explain why the apparently-benevolent act of handing someone a gift is driven by a deeply ingrained evolutionary-conditioned selfishness. “Gift giving goes back to a long time before Christmas and it’s always done with some expectation. You don’t just give a gift for no reason, you do it because you expect something back in return,” says Kimberley Hockings, a lecturer in biosciences at the University of Exeter.

“Even a present from Santa is tied to good behaviour – if you’re bad, you don’t get anything. The reasons are varied in humans because there are so many cultures that do things in different ways. There doesn’t necessarily have to be a gift in return – it could be about love, affection, or social bonding,” she adds. But humans only account for a tiny proportion of global present giving, since creatures across the animal kingdom – insects in particular – are constantly bearing gifts. But the reasons for this are typically much more straightforward, with the male giving the female a present – normally in the form of food – to increase his chances of “mating access”, says Dr Hockings.

The Virgin Birth:
With the exception of Mary, virgin births, or parthenogenesis, have not historically been observed in humans or mammals – although the development of IVF techniques in which an egg is fertilised by sperm outside the body, has changed that in recent years. But elsewhere in the animal kingdom parthenogenesis is far more common. Insects do it all the time – notably some species of bees and wasps – while in 2006 scientists observed two separate cases of isolated female Komodo dragons producing offspring without fertilisation by a male. This suggests the female of the species can switch between sexual and asexual reproduction depending on the availability of males. Boa constrictors have also been known to reproduce without a male while at least one species of lizard, the New Mexico whiptail, has no males at all in its population.

Star of Bethlehem:
Leaving aside divine occurrences, there are four explanations for the transient bright “star”. A bright meteor: This would capture the attention, but it would be too transient to guide the three wise men on a long journey to the Roman province of Judea. A bright comet: This might be interpreted as a star and it could be widely visible over the Earth for many weeks. Furthermore, ancient Chinese records have two possible sightings of comets at about the right time. However, the ancient world regarded comets as a sign of doom – making it unlikely they would be viewed as heralding the birth of a messiah. A nova or supernova: A flaring or exploding star would fit the bill. There is some record of a nova in 5 BC in Aquarius or Capricornus, but it was not particularly bright or noteworthy. A planetary conjunction: The apparent close approach of two bright planets is perhaps the most promising explanation. Astronomers know a rare “triple alignment” of Jupiter and Saturn occurred around the time of Jesus’ birth when the planets approached each other closely three times between April and December.

The festive spirit:
The joy surrounding Christmas can increase the level of dopamine and serotonin in your brain, chemicals which affect your happiness. Dopamine drives us to seek pleasure and reward while serotonin is thought to increase our feelings of worth and belonging. So when people talk about “Christmas cheer” they may be onto something, says Kira Shaw, a neuroscience postdoctoral researcher at the University of Sussex.

Santa’s busy night:
Scientists have traditionally marvelled at the extraordinary feat of delivering a billion-plus presents around the world in just over 30 hours (time differences and the rotation of the Earth give him a little longer than 24 hours). But the real heroes are the reindeer, according to Jason Chapman, a movement ecologist at Exeter University. “People talk about the journey that Santa does but it’s the reindeer that do all the hard work. So I wanted to compare the reindeers’ performance over a single non-stop flight with some of the most extreme animal migrations,” he said.

Dr Chapman conservatively estimates that the reindeers cover around 50,000 miles on their big night as they zig zag across the globe – giving them an average speed of 1,700 miles per hour, or three times the speed of a jumbo jet. By contrast, the next most impressive traveller in the animal kingdom is the Arctic Tern, which flies 60,000 miles – but it spreads its journey over a year and takes breaks.

Loneliness:
There is nothing like Christmas to underline a sense of loneliness. At a time when everyone else seems to be cuddling up with their family around the TV, millions will suffer alone. And that is not healthy. Only this month, a new study found that loneliness – which afflicts nine million adults in the UK – is as harmful as obesity or smoking 15 cigarettes a day, raising the risk of premature death by a third. “Social isolation can have a devastating impact not only on people’s mental wellbeing. NHS staff see first-hand the acute consequences of loneliness, which can affect so many people of all ages,” said Professor Jane Cummings, Chief Nursing Officer for England.

Stress:
For many people Christmas is extremely stressful thanks to all that last minute shopping in busy malls and attempts to cook a dry old bird for a disappointed family. Stress can exert a physical response in your body, with the automatic release of adrenaline – leaving you feeling exhausted.

Why we need the Christmas myth:
Christopher Boyle, University of Exeter and Kathy McKay, of the Univerisity of Liverpool, have written an essay on just this subject. “Adults who propagate the Santa Claus myth are able to go back to a time when they believed that magic was, indeed, possible. By perpetrating the Christmas lie, one believes in the possibility of a better place and time,” they wrote in The Lancet. “It seems that by returning to a fantasy world, there is a comfort in being able to briefly re-enter childhood, which was a magical experience for many. A time when imagination was accepted and encouraged but which becomes lost in the space and time of adulthood. The self-conscious recreation of myth seems to be as popular as it ever was. Might it be the case that the harshness of real life requires the creation of something better, something to believe in, something to hope for in the future or to return to a long lost childhood a long time ago in a galaxy far far away…?” they ask.