The sky is powder blue, and the sun magnificent, as I stride through glittering grass and fallen sycamore seeds to Dowth, a Neolithic passage tomb in County Meath. Unlike its more famous neighbour, Newgrange, there are no tour buses here, no glitzy visitor centre, and – apart from today – no public access; only a wooden stile and a small sign on the verge of an Irish country road.

The mound of the large burial chamber rises from the earth like a pregnant belly. At its base, I instinctively turn left, walking clockwise – Sunwise – around it, until I come to a large boulder bearing ancient markings. The seven suns etched into its surface are just as a child would draw them, with rays radiating from a central circle. Pecked out with a hammer and stone chisel some 5,200 years ago, they’re a clue to the phenomenon that occurs here on this, the shortest day of the year.

Our ancestors revered the Sun as a creator and destroyer of life. Their senses told them that when the Sun is absent, everyone and everything suffers. They tracked its movements, noticing how it rises a little further along the horizon each day, until the solstices, when it pauses (the word solstice comes from ‘sun standstill’), then tracks back in the opposite direction. The winter solstice was particularly significant. To mark this crucial turning point, when the Sun appeared to be at its weakest, people held feasts and created monuments, which they aligned with the rising or setting midwinter Sun, perhaps in the hope that things would get better: that the barrenness of winter wasn’t forever.

Today, we’ve largely lost this connection. Electric lighting and central heating buffer us against the changing seasons, and enable us to work and socialise around the clock, even during the long nights of winter. Where our ancestors spent most of their days outside, we live approximately 90 per cent of our lives indoors.

Yet we feel the sway of the Sun on our bodies, nonetheless. At the top of the mound, I meet four women who invite me to join their picnic of chicken wings and Buckfast – a sweet, caffeine-infused, fortified wine. For them, this trip is an annual pilgrimage: at a time when Christmas has become so consumer-driven, they relish the simple act of sharing a picnic in the sun as a powerful way to reconnect with the seasons and put things back into perspective.

One of them, Siobhan Clancy from Tipperary, tells me: ‘Just sitting with the sun in my eyes, I feel like there’s something in my lizard brain that’s saying: “Yes, there’s sunlight; you’re alive; you’re awake; you’re getting through winter, and everything’s turning again.” We’ve become so detached from the nature around us. Just to be here and really experience the conditions of winter and that beautiful low, pale silver-gold light; to be bathed in it feels so lovely.’

Sunlight does numerous things to our bodies: it enables us to make vitamin D, and it keeps our circadian rhythms – 24-hour peaks and troughs in the activity of pretty much every biological process – synchronised with the time of day outside. It tweaks our immune and cardiovascular systems, too. Our blood pressure is lower in summer than in winter, for example, since sunlight triggers the release of nitric oxide from our skin, causing our blood vessels to relax and widen.

There are also measurable differences in our brain chemistry across the seasons. Levels of serotonin, the mood-regulating neurotransmitter, are highest in summer and lowest in winter, as is availability of the amino acid L-tryptophan, which is needed to synthesise it.

When the UV rays in sunlight hit our skin, we release endorphins – the same chemicals that trigger a runner’s high. Sunlight boosts alertness, which might be another reason why a bright winter morning feels so uplifting. In fact, exposure to around an hour of blue spectrum light – abundant in sunlight – boosts our reaction speeds to the same extent as drinking several cups of coffee.

Our connection with sunlight goes deeper still. The evolution of photosynthesis in the Earth’s early oceans was responsible for transforming the planet’s atmosphere into the hospitable place it is today. As plants and algae harness the Sun’s rays to create energy, they release oxygen. The life-sustaining air we’re breathing now is largely a product of sunlight. So is the food we eat, because plants can’t survive without sunlight, and we depend on plants – or on plant-eating animals – for our survival. With every bite eaten and breath taken, we incorporate sunlight into the fabric of our bodies.

Looking out over the Boyne Valley in Ireland, I spy the shell of an old church, and I’m reminded that Christmas is just a few days away. This midwinter festival also has echoes of Sun worship. ‘It was a custom of the pagans to celebrate on the same 25 December the birthday of the sun at which they kindled lights in token of festivity,’ wrote the Christian writer Scriptor Syrus in the late-4th century. ‘In these solemnities and revelry the Christians also took part. Accordingly when the doctors of the Church perceived that the Christians had a leaning to this festival, they took counsel and resolved that the true Nativity should be solemnised on that day.’

Once you start looking for solar imagery in Christian churches, you see it everywhere: in the haloes of angels and the circle of the Celtic cross. Many churches in the British Isles are orientated East, towards sunrise. As the historian Ronald Hutton at the University of Bristol told me: ‘That extraordinary moment of the return of the light, and the Sun heaving above the horizon is one of the most dramatic daily phenomena in nature. If you are backing a religion which embodies the sense of hope and new life and regeneration and resurrection, then to face the course of the Sun seems an obvious symbolism.’ In other religions and cultures, too, light symbolises goodness and knowledge. Light conquers the darkness; it brings hope and rebirth. Light is truth; we are enlightened.

While the women at Dowth pack up their picnic, I take another swig of Buckfast. The sweet liquid trickles down my throat, and I feel a swell of cheer, which isn’t entirely due to the alcohol. Something about this act of breaking bread with strangers in the dazzling midwinter sunlight is undeniably uplifting. The Sun is now lower in the sky, and it is time to make our way inside this tomb and witness the spectacle that occurs at sunset. The mud is churned up around the small stone entrance at its base, and the modern iron gate is pushed back, daring us to step inside. I stoop to creep down the narrow passage, stumbling blind into perfect darkness. As I trip on a rounded stone, a gloved hand grabs mine and pulls me leftwards, into a pitch-black chamber.

As my eyes adjust, I start to make out other human forms, including those of Siobhan and her friends. The chamber we’re standing in is circular, and lined with large stone blocks, some of which are engraved with Neolithic art. To the right is a second, smaller chamber, where people with torches are examining some of these symbols. Despite being a refuge of the dead, it’s surprisingly warm inside, welcoming in feel, as if we really are inside the belly of the Earth.

At 2pm, the event we’re waiting for begins. A shaft of sunlight from the passageway begins to penetrate the chamber. The light has a golden quality, and forms a long rectangle on the floor, which grows then slowly creeps backwards as the Sun sets lower in the sky. At 3pm – about an hour before sunset – the sunlight hits a series of large stones lining the back wall, illuminating a profusion of pecked marks, clustered into cup shapes, squiggles and Sun-like spirals. One of the stones curves outwards, reflecting the sunbeam into another wedge-shaped recess, where a solar ‘wheel’ and spiral are carved. At 3.30pm, the sunlight begins to retreat from the chamber, plunging us back into darkness.

This phenomenon occurs at Dowth from late-November to mid-January, but the strongest illumination occurs on the solstice, when the Sun is at its lowest. We can only speculate about what our ancestors had in mind when they built this place. Possibly, this sight wasn’t intended for the living at all, but rather a signal to the dead that it was time to leave their tomb. Certainly the journey through the dark tunnel, back into the light, feels a lot like rebirth.

Stepping back outside, I look at the glowing orb being gobbled up by the horizon. Tomorrow it will rise again, a little stronger, and the next day, stronger still. Summer might yet be half a year away, but it is coming, and this sticky mud I’m standing in will dry and sprout green shoots. It’s reliable, that big old burning ball of gas up in the sky. Hail to thee, our nearest star.