IF EVER YOU NEED a reminder of the sun’s awesome power, the Mojave Desert is a good place to start. In summer, when the daytime temperature frequently hits 49°C (120°F), stepping outside feels like opening the door to a giant furnace.
The local flora and fauna arm themselves against this heat: Joshua trees grow tough, concave spikes for leaves, minimising water loss and channelling what little rain does fall down towards their trunk and roots; jackrabbits develop enormous ears with shallow blood vessels that allow their body heat to quickly evaporate. Other creatures are nocturnal or emerge only at dawn or dusk to avoid the sun’s heat; while yet others, such as the desert tortoise, sleep out the entire summer in underground burrows. Then there are vultures, which cool off by peeing onto their own legs.
Humans are less well equipped for such harsh conditions. In the Sonora Desert immediately to the south, hundreds of Central American migrants meet their deaths each year while trying to cross the border into the USA, the sun stripping away their body fluids and causing them to overheat.
Yet the sun’s power also creates opportunities. Plants harness its rays to generate food, while shimmering solar farms are springing up, bent on transforming those rays into electricity. At the largest of these – the Ivanpah Solar Plant, 45 miles south-west of Las Vegas – a glittering sea of sun-tracking mirrors captures and focuses sunlight onto three boiler-topped towers, which drive turbines that supply electricity to hundreds of thousands of homes. Pity any bird crossing the path of these concentrated sunbeams, though: they’re known as ‘streamers’, referencing the wisps of white smoke left in their wake as they are instantly incinerated. Across the ages, in civilisations separated by thousands of miles of land or ocean, people have revered the sun as both a creator and a destroyer – a relationship that continues to this day.
In Las Vegas, though, which rises out of this hostile landscape in defiance, the sun has been dethroned. At night, the neon-soaked Strip is reportedly the brightest place on earth, while the strongest artificial light on the planet emits from the tip of the Luxor Resort and Casino’s glass and steel pyramid: each night, it sends this powerful ‘sky beam’ upwards, as if issuing a direct challenge to our nearest star. On a clear night, airline passengers can see the beam from some 275 miles away, and their pilots use it to help them navigate. The artificial light also confuses the navigation systems of insects, luring them to their deaths: the concentrated swarms provide a buffet for bats, which are in turn feasted upon by swooping owls.
Elsewhere in Vegas, realising the power it wields over our minds and spirits, resort owners have deliberately banished the sun from their casino floors. The 24-hour cycle of light and darkness is crucial to our internal sense of time; if there are no windows, it’s easier for gamblers to lose track of it and stay for hours longer than they mean to – particularly if artificial light is deployed to wake them up. Some casinos even go so far as forbidding dealers from wearing a watch, so that if anyone asks the time, they can’t say. Chairs are ergonomically designed to allow players to sit for hours in comfort, while oxygen is pumped in to boost alertness.
In this twilit world, artificial light reigns supreme, and it can have a huge effect on us: strategically placed spotlights draw consumers towards jangling, flashing slot machines; but the colour of the lighting can also be deliberately tweaked to manipulate people’s behaviour. Blue-white light simulates daylight and makes people feel more alert, which can fool them into lingering longer at tables and slot machines. Meanwhile, red light can raise our level of physiological arousal: a study has shown that people gamble more money, place more bets and choose riskier options under red light, compared to blue. Pair that red light with fast music, another study showed, and people will bet faster on roulette.
Some time ago, I found myself in the middle of this muddled-up world, while covering a conference for New Scientist magazine. Giddy with jet lag and having spent the entire day in a windowless meeting room, I was desperate to spend the few spare hours I had soaking up some sunlight. It was October – which meant that the sun’s ferocious heat was muted somewhat – and the desert sky was cloudless, yet the entire city seemed set up to hide this fact, with chains of underground malls linking one hotel to the next so that you never have to step outside.
Finally, I found myself surrounded by mock-Greco-Roman architecture in the labyrinthine mall of Caesar’s Palace, glimpsing what appeared to be daylight up ahead. My excitement was quickly deflated as I drew closer and looked up: above me soared an impressive – yet completely artificial – sky. As I slumped, defeated, next to a replica of Rome’s Trevi Fountain, it struck me just how perverted our relationship with natural light has become.
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Our biology is set up to work in partnership with the sun. Life itself arose on earth because its relationship with the sun was a special one. Earth’s distance from the sun, not too close and not too far away, meant that the water on its surface remained liquid, whereas on Venus it was baked away, and on Mars it was locked up as ice. Sunlight-driven reactions may also have provided the molecular raw materials necessary for the evolution of life in these early oceans. Some 1.4 billion years later, tiny single-celled organisms called cyanobacteria had evolved, which clumped together, forming brilliant blue-green rafts. And though they were individually tiny, they achieved extraordinary feats: taking the sun’s light and turning it into chemical energy through photosynthesis, which they stored away as sugar, thus incorporating sunlight into their very being. In the process, they produced oxygen, which accumulated and transformed earth’s atmosphere into the hospitable place we know today.
Life flourished and grew more diverse, evolving and changing until, after another 2.4 billion years, the human species branched off. As we feasted upon the abundant plants and animals, and walked under the sun’s rays, we too assimilated starlight into the very fabric of our beings. For every plant we ate depended on the sun’s energy to grow, as did every animal: these creatures couldn’t survive without eating plants – or eating animals that ate plants.
And as the sun’s light penetrated our eyes, it changed the chemistry of our brains, tweaking pathways that control our internal sense of time. So the sun brought order to our ancestors’ biochemical reactions and behaviours and, as they looked up at the sun and at the pinpricks of light that decorated the heavens, they found that it also brought spiritual order to their lives.
Little wonder then that humans have long worshipped and revered our nearest star: from the Stone Age solstice worshippers of Britain and Ireland to the Inca who believed they were descended from the sun god Inti. Our histories, religions and mythologies are packed with solar symbolism – whether it’s the Greek god Helios pulling the sun across the sky in his chariot; the ochre-hued Sun Woman in the mythology of the Aboriginal people of northern Australia, carrying her torch across the sky; or the significance of light and rebirth in Christianity.
This makes sense, because from humanity’s very beginnings, the sun has governed both our bodies and our experience of the world. To our ancestors, the daily rising and setting of the sun, and the seasonal fluctuations in heat, light and food must have seemed extraordinary, not to mention life-changing.
Imagine yourself as a Stone Age man or woman. There are no calendars telling you what time of year it is; no almanacs to explain what’s gone before. You don’t know that the world is round, tilted and spinning, and that it revolves around the sun, which is just one of billions of vast fiery balls floating in a vacuum called space. And not knowing any of this, neither do you understand that the sun will continue to rise and fall each morning, and the seasons turn, until – in approximately five billion years’ time – our sun will burn out completely, but not before it dramatically expands and strips the earth of its water, leaving a dead and barren planet in its wake.
Instead, you look to the heavens and imagine a moving cast of characters, each with a story to tell: a great bear; a chained lady; a hero; a water snake. But, most of all, you revere the biggest and brightest of these celestial bodies: the sun, and its cool, pale companion, the moon. Your senses tell you that when the sun is close and present, plants grow, animals reproduce, and you feel warm and happy. When the sun goes away, everyone and everything suffers.
To you, the sun must seem to have a will of its own; one that could potentially be influenced by your own actions. Because of this, you track its movements, noting where this powerful being rises and falls each day. Its regular disappearance and magical rebirth each morning chime with your own observations of human death and birth; perhaps its circularity gives you hope that we will similarly be reborn one day.
Particularly in a place like northern Europe, you will have watched the sun moving a little further along the horizon each day, as if departing, and associated this with the growing cold, shrinking light, and the withering of your crops to nothing. Finally, for a few short days during the coldest, darkest, deadest time, the sun stops still in its tracks, almost as if it’s reconsidering its path (the word ‘solstice’ translates as ‘sun standstill’). Possibly, there’s an opportunity to win back its favour. If the sun returns, your seeds will grow; your cattle, pigs and sheep will bear new offspring for you to fatten and consume; and your children will survive. This has happened before, but there’s no guarantee that it will happen again.
You don’t hold back: people gather from far and wide, animals are slaughtered, and you hold a giant feast; your elders conduct elaborate ceremonies focused around the sun. In the darkness, there springs hope: that the light will return, and life will be reborn from the wastelands.
Archaeological evidence of our ancestors’ preoccupation with the solstices – and particularly the winter solstice – has been uncovered at numerous sites, including New-grange in Ireland, Stonehenge in southern England, Machu Picchu in Peru and Chaco Canyon in New Mexico.
But our ancestors not only revered the sun from a spiritual perspective – they knew that it could be harnessed to promote health. Romans, Greeks, Egyptians and Babylonians all recognised that the sun had powerful curative properties.
Almost 4,000 years ago, the Babylonian king Hammurabi was advising his priests to use sunlight in the treatment of illnesses. Similar beliefs were held in ancient Egypt and India, where skin diseases like vitiligo, which causes the skin’s pigment cells to be destroyed, were treated by applying plant extracts and then exposing the affected area to the sun. Our ancestors clearly noticed that sunlight has the power to transform seemingly mundane substances, such as ground-up plant leaves, into agents that can heal.
Such ‘photodynamic therapy’ has recently been rediscovered, and some skin cancers are now treated by applying a photosensitising agent to the affected area: when light is shone on it, a chemical is formed that kills the cancer cells. Photodynamic therapy is also increasingly used to treat acne. Meanwhile, modern skin clinics use UV light without a photosensitising agent to treat conditions such as eczema and psoriasis because it suppresses inflammation.
Our ancestors used sunlight as a tonic for non-skin-related conditions as well. The Ebers Papyrus – an Egyptian medical document dating to around 1550 BC – advised anointing and exposing painful parts of the body to sunlight. This chimes with very modern research looking at how the sun’s rays affect us: beyond UV light, the sun emits light from across the spectrum, including the visible wavelengths that become most obvious when sunlight hits a raindrop, and infrared light. Light from both ends of the spectrum can influence pain perception: infrared light is now used as a treatment for various types of acute and chronic pain and is currently being investigated to promote wound healing. UV light also stimulates the production of endorphins, which blunt our perception of pain.
The Greek doctor Hippocrates, who is often referred to as the father of modern medicine, similarly recommended sunlight for the restoration of health. He promoted sunbathing and constructed a large solarium at his treatment centre on the Greek island of Cos. Hippocrates believed that sunlight could be beneficial in the treatment of most diseases, although he warned against excessive sun exposure, pleading for moderation – wisdom that still holds true today. In fact, the first accredited description of the deadly skin cancer melanoma came from Hippocrates: its name derives from the Greek words, melas, meaning dark, and oma, meaning tumour.
Hippocrates also laid the foundations of ‘clinical observation’, believing that closely watching a patient and recording their symptoms was a critical part of medical care. It was this attention to detail that led him to observe the first recorded example of a daily rhythm in humans besides sleep: a 24-hour ebb and flow in the severity of fever.1
Like ancient medics in India and China, Hippocrates also saw the turning of the seasons as important to human health: ‘Whoever wishes to pursue the science of medicine in a direct manner must first investigate the seasons of the year and what occurs in them,’ he wrote.2
Believing that disease arose from an excess or deficiency of four bodily fluids – phlegm, blood, yellow bile and black bile – Hippocrates argued that seasonal changes in these ‘humours’ explained peaks and troughs in different illnesses at various times of year. He advised people to adapt, according to the seasons, what they ate and drank, the type of exercise they took, and even how often they had sex, in order to keep these humours in balance.3
Another celebrated ancient Greek physician, Aretaeus of Cappadocia, recommended that ‘lethargics’ be exposed to sunlight, while the Roman physician Caelius Aurelianus wrote that light as well as darkness could be used as a medical treatment, depending on the condition. Solaria also featured in many Roman homes and temples, and sunbathing was particularly advised for epilepsy, anaemia, paralysis, asthma, jaundice, malnutrition and obesity.
Although there is no record of any clinical trials demonstrating the effectiveness of such methods, today we can see some plausible mechanisms by which sunlight exposure might have exerted a therapeutic effect. For instance, we know that the sun enables us to manufacture vitamin D in our skin, and that levels of it vary over the year; several studies have linked vitamin D deficiency to epileptic seizures and anaemia. The bone disorder rickets is also caused by a lack of vitamin D, while supplements of it have been shown to help prevent upper respiratory tract infections and the worsening of existing asthma.
Phototherapy is widely used to treat jaundice in newborn babies; light in the blue-green part of the spectrum breaks down the bilirubin pigment in the blood that causes it. Meanwhile, conditions associated with lethargy, such as insomnia and depression, as well as obesity, have been linked to a disrupted body clock, and regular exposure to daylight – particularly first thing in the morning – can strengthen these daily rhythms. Sunlight exposure also boosts the availability of the mood-regulating substance serotonin in the brain, while darkness is being investigated as a treatment for mania.
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Seasonal and daily fluctuations of light and dark – and their impact on our bodies – are increasingly being investigated and accepted by modern scientists as well. We live in a world that is vastly different from the one inhabited by our ancestors, and our lives bring pressures to bear which have a significant impact on our well-being. Humans have evolved to synchronise sleep with when it’s dark outside and we’re usually at our most active when the sun is up. As anyone who has worked a night shift or flown a long distance and contended with jet lag will know, this isn’t a set-up that we can easily override: it’s very difficult to sleep when your body thinks it should be awake, and vice versa. But sleep is just the tip of the iceberg. The body is a very different place during the day compared with the night: the kidneys are less active at night, which means that we produce less urine and need to pee less; core body temperature is lower, as are our reaction speeds; and our immune systems respond differently to invaders. Then, as the sun comes up, and day begins, blood pressure and body temperature rise; hunger hormones kick in; and our brains and muscles shift into a higher gear.
These daily fluctuations in our biology are called circadian rhythms – and they are as important to us as they are to the desert coyotes and rattlesnakes that only become active once the sun is low or vanished from the sky: they are the reason we feel so terrible when we’re jet-lagged, or start to yawn once the sun has gone down. By tweaking our urges, behaviour and biochemistry, they prepare us for regular events in our environment, like mealtimes or getting up in the morning, which are themselves dictated by the daily cycle of light and dark. Sunlight, and its absence at night, are the main mechanisms we use to synchronise these internal rhythms to the external time of day. If we don’t see enough daylight, or we’re exposed to too much artificial light at night, our bodies become confused and no longer work as efficiently.
Circadian rhythms begin to develop in the uterus, but those governing sleep don’t fully develop until several months after birth. This makes sense: newborn infants need to feed little and often, and a prolonged period of consolidated sleep would interfere with this. Even so, infants receive chemical time cues via their mother’s milk, which promote sleepiness during night-time, while infants who are exposed to more bright daylight also sleep more soundly at night.
In adults, there are daily rhythms in body temperature, physical strength, mental alertness, the secretion of various hormones, and many more things besides.
Sunlight doesn’t only affect the body clock: it affects our physical and mental health in other ways as well. Most of us are aware that we need regular sun exposure for vitamin D production, which is essential for building a healthy skeleton, but scientists are now discovering new, and astonishing, health benefits to being outside. Mounting evidence suggests that our sun exposure over a lifetime – even before we were born – may shape our risk of developing a range of different illnesses, from depression to diabetes. Recent studies have shown a protective relationship between sun exposure and multiple sclerosis, as well as one with childhood short-sightedness. Being out in the sun, we are starting to understand, can lower our blood pressure, calm our immune system and even alter our mood. Even without such knowledge, most of us are instinctively drawn to sunlight because sitting in it just feels so great, and there may be a reason for that: when the sunlight hits our skin, our bodies release endorphins, the same ‘feel good’ hormones that produce a runner’s high.
There are good reasons why we feel depressed or anxious when we’re cut off from the sunlight. As I lurched, like a confused moth, through underground malls and vast casino floors, my sense of time becoming ever more distorted, I considered how we crave the sun in the depths of winter or if we spend too long sitting indoors; how even on gloomy days, taking a walk outside is often a great tonic. And my mind turned to how a distorted relationship with sunlight can affect, and even harm, our health.
Las Vegas is an extreme example, but most of us have a weaker relationship with the sun than our ancestors ever did. While they were exposed to extremes of light, dark, heat, cold, feast and famine, triggered by the sun, we shield our bodies from sunlight in the daytime, and – thanks to electric light bulbs, screen time and central heating – expose ourselves to an artificial version of it in the evening. This removes many of the natural cues that tell our bodies that it’s time to sleep. And because we’re more active in the evenings, we’re often eating our largest meal of the day when we’re physiologically least prepared to deal with it. At the same time, alarm clocks and nine-to-five office hours are waking us up before our bodies are necessarily ready. Besides making us feel groggy and irritable, chronic sleep deprivation is emerging as a major cause of ill health. We need sleep to both mentally and physically recover from daily life: the ubiquity of artificial light at night is robbing us of one of the best preventative medicines going.
Meanwhile, dimly lit offices, sunscreen and indoor living all mean that we’re depriving ourselves of the UV light our skin needs to synthesise vitamin D, and – as scientists are increasingly discovering – to tweak our immune systems and help regulate our blood pressure. It also means that we’re failing to reap the mood-boosting benefits of sunlight.
But at least nine-to-five office hours are roughly synchronised to the day/night cycle. In 2007, when I travelled to Las Vegas, the International Agency for Research on Cancer added night-shift work to the official list of ‘probable’ human carcinogens. Being exposed to bright light at night, as both shift workers and casino clients are, forces the body to feel alert when it should be sleeping, setting off a cascade of damaging effects. Shift work – and increasingly bright light at night in and of itself – has been linked to a host of conditions, including heart disease, type 2 diabetes, obesity and depression. Some academics have even suggested that artificial light may be why these conditions have risen to epidemic proportions in modern life. Another theory as to why shift work is associated with so many illnesses is that it encourages us to eat when our bodies think we should be sleeping, which confuses our internal rhythms even further.
Over the past two decades, there has been a scientific revolution in the field of chronobiology – which studies these cyclical changes in our bodies – with the vital importance of our biological relationship with our nearest star becoming ever clearer. In 2017, the Nobel Prize for medicine was awarded to circadian biologists, in recognition of just how important this relationship is to human health. Almost half of our genes are under circadian control, including ones associated with every major illness investigated so far – including cancer, Alzheimer’s disease, type 2 diabetes, coronary artery disease, schizophrenia and obesity. Disrupting these rhythms – as we do when we sleep, eat or exercise at the wrong time – is associated with an increased risk of many of these diseases, or a deterioration of symptoms associated with them. What’s more, many of the drugs we rely on in modern medicine target biological pathways that are regulated by circadian clocks, which means that they could be more or less effective, depending on when we take them. Meanwhile, side effects associated with radiotherapy and several chemotherapy drugs used to treat cancer can be significantly reduced by delivering them at a time when the healthy cells, which they also damage, are resting.
Our physical relationship with the sun has implications for even the most fit and healthy people as well. World-class athletes are employing circadian biologists to optimise their physical performance, while NASA and the US Navy are applying this astonishing science to keep astronauts and submariners in peak mental shape during their shifts and help them overcome jet lag more quickly.
And it’s not only sunlight. Increasingly, we’re learning how we can harness the power of artificial light to boost our alertness and physical health rather than undermine it. As we age, our circadian rhythms start to flatten and become less pronounced, so researchers are investigating whether artificial light could be used to supplement daylight in care homes, strengthening these rhythms and alleviating some symptoms of dementia. Hospitals are using circadian-inspired lighting to boost people’s recovery from stroke and other serious illnesses, while some schools are using it to boost pupil’s sleep, daytime alertness and exam grades.
A better understanding of our relationship with light could improve multiple aspects of our health, both mental and physical: this book will teach you what you need to know, and what you can do to strengthen your own circadian rhythm, optimise your sleep and performance, and hasten your recovery from jet lag. It will also reveal the other health-promoting properties of sunlight, and how to balance this against its damaging effects.
Forging a healthier relationship with light doesn’t mean we have to ditch our electronic gadgets and return to the dark ages. But we do need to acknowledge that excessive light at night and an absence of bright light during the day is harmful and so take steps to counter it. We evolved on a rotating planet, when day was day and night was night: it’s time to reconnect with those extremes.
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For millennia, then, people have looked at the sun as crucial to our health and seen in its daily and annual cycles a key to understanding the cosmos, yet it is something we seem to ignore or forget about in our modern, daily lives.
Hippocrates would have encouraged us to observe seasonal changes in our mood or energy levels and adjust our behaviour accordingly. However, the relative comfort of our homes and offices, together with the demands of our economic system, encourage us to maintain the same work schedule year-round. We’re also expected to maintain a similar level of sociability. Winter is viewed as a gloomy inconvenience and, rather than getting outside to reap what little daylight there is, we switch the lights on and crank up the central heating instead. This may be detrimental to our mental health: exposure to bright light, particularly during the early morning, is a tried and tested way of combating the winter blues. Similarly, many of us keep the lights and heating on long after the sun has set and spend these already brightened evenings in front of electronic gadgets, which produce yet more light. This may undermine our ability to get a decent night’s sleep.
The ancients were right to put the sun at the centre of their world. Sunlight was essential for the evolution of life on earth, and it continues to influence our health today. But darkness is also important: the natural cycle of night and day that the sun presides over is implicated in everything from our sleep patterns, to our blood pressure, to our lifespans. Denying access to this cycle, as we do when we cosset ourselves indoors and spend our evenings under bright artificial lights, could have far-reaching consequences that we’re only just beginning to grasp.