How getting more daylight can improve your mental and physical health.
Linda Geddes
Spending less time outside and more time in dim artificial light is disrupting our body clocks and undermining our health. The good news? A little daylight goes a long way.
MORTEN HALMØ PETERSEN used to live in a windowless basement flat in Copenhagen. If he didn't get out in the daytime, he would lose track of time and start becoming irritated and depressed. "When you are living in a basement with only artificial light, it becomes very clear that something is lacking," he says. "It's an emotional, physical and mental thing all combined."
You can say that again. Our lifestyles have rapidly changed our relationship with light. Prior to the invention of gas lighting at the turn of the 19th century, the only artificial light we could rely on was from flickering firelight, candles or whale-oil lamps. People also spent many more of their waking hours outside.
Today, the average Westerner spends 90 per cent of their life indoors. That means we are getting less light during the day and being exposed to more light at night. This pattern is increasingly being linked to disrupted sleep and circadian rhythms – 24-hour fluctuations in our biology and behaviour – with consequences for our physical and mental health. Meanwhile, getting too little sunlight is contributing to vitamin D deficiency and may be undermining our immune and cardiovascular systems as well.
Our changed relationship with the sun is profoundly affecting our biology. That's why people like Petersen are being recruited by researchers to help investigate how much damage we do by shying away from the light, and just how much light we need. The good news is researchers are finding that even small increases in your exposure to bright light during the day have a wide range of benefits, from improving sleep and mood to speeding recovery from serious illness.
Today, most of us spend our daytimes in the equivalent of twilight. Illuminance is measured in lux, which refers to the amount of light striking a surface (see “The light in our lives”). Studies of the Amish, who lead an off-grid and more agrarian existence, highlight how much our relationship with light has changed. In summer, the Amish are exposed to an average daytime illuminance of 4000 lux, compared with 587 lux for the average person in the UK. In winter, that drops to just 210 lux in the UK, compared with 1500 lux for the Amish. During the evening, the average illuminance in Amish homes is around 10 lux – up to five times lower than evening light levels in electrified homes.
At the back of the eye, behind the rod and cone cells that enable our brains to construct images, are light-responsive cells called intrinsically photosensitive retinal ganglion cells (ipRGCs). These fire in response to any light if it is bright or sustained enough but they are particularly sensitive to light in the blue part of the spectrum. This includes bright daylight, many LEDs and light from screens.
These ipRGCs send signals to areas of the brain that control alertness. One study found that exposure to an hour of low-intensity blue light increased people's reaction speeds more than if they had drunk two cups of coffee. The cells also send signals to a tiny patch of brain tissue called the suprachiasmatic nucleus (SCN). This functions as the body's master clock, tweaking the timing of the circadian clocks ticking in every cell of our bodies to keep them synchronised with each other and with the external time of day.
Exposure to light in the evening delays the timing of the SCN, making you more owlish. "One problem with light at night is that it makes you want to go to bed later, but you still have to get up at the same time, so you get shorter sleep," says Mariana Figueiro, director of the Lighting Research Center at Rensselaer Polytechnic Institute in New York state. Bright light exposure in the morning advances our body clocks, making us more lark-like.
It is also important that we are plunged into darkness at night. Besides shifting the timing of our body clocks and providing an unwanted boost to alertness, light at night also suppresses the release of melatonin, a hormone that reinforces the biological message that it is night, including to parts of the brain that promote sleep. A 2016 study found that people living in areas with elevated levels of light pollution tend to go to bed and wake up later than those living in darker areas. They also sleep less, are more tired during the daytime and are less satisfied with the quality of their sleep. That is a worry when the area of Earth's outdoors that is artificially lit is increasing by more than 2 per cent a year, according to a recent study of satellite images.
Extremes of darkness and brightness are key to our biological rhythms.
Experiencing a marked contrast between day and night is important. Our light exposure influences the amplitude of our biological rhythms: they become flatter under more constant light conditions, which has been associated with poorer sleep. So what can we do about it?
The General Services Administration is an independent agency of the US government that oversees the management of government-owned buildings, making it the largest landlord in the US. Its bosses wanted to establish whether designing more daylight into their buildings affected the occupants' well-being, so they asked Figueiro to study it.
The results were initially disheartening: just a metre or so away from the windows, the illuminance dropped sharply. When Figueiro compared office workers' sleep, she found that staff who got more bright light during the day – perhaps because they sat next to a window, walked to work or spent their lunch breaks outdoors – fell asleep faster at night and slept for longer than those who got less light. Workers exposed to more bright daylight between 8 am and noon took an average of 18 minutes to fall asleep at night, compared with 45 minutes in the low-light exposure group. They also slept for around 20 minutes longer and had fewer sleep disturbances.
Such improvements have been recorded elsewhere too. Dutch researchers fitted 20 people with devices to record daytime light exposure and then assessed their sleep on subsequent nights. Greater daylight exposure was found to be associated with less fragmented sleep and more deep sleep, which you need to feel refreshed in the morning. Also, "if they did wake up at night, they were less sleepy the next morning", says Marijke Gordijn at the University of Groningen.
There are other benefits as well. In the General Services Administration study, greater daylight exposure was associated with lower scores on a self-rated scale of depression. That is consistent with other findings that bright morning light can help treat seasonal and other forms of depression. That was assumed to be because light strengthens body clocks, nudging them earlier, which helps to improve quality of sleep by becoming more aligned with when we have to wake. But the link may be more direct: a recent animal study showed that the same ipRGCs that feed into the brain's master clock also connect to the thalamus, a brain area related to mood. "It is a hugely important finding," says Katharina Wulff, a daylight researcher at Umeå University in Sweden. "It shows that light not only affects the clock, it can directly act on mood."
The light in our lives
Then there is alertness. A 2017 German study suggested that exposure to bright light in the morning boosted people's reaction speeds and maintained them at a higher level throughout the day – even after the bright light had been switched off. It also prevented their body clocks from shifting later when they were exposed to blue light before bed. "The effects of light in the evening highly depend on the light you had in the morning," says Dieter Kunz of the Charité University Hospital in Berlin, who led the study.
People in glass houses
Some researchers are taking it to extremes. Petersen, in a reversal of his basement flat days, is one of 12 volunteers who spent three nights on a Danish island in a glass dwelling designed to provide inhabitants with full exposure to the 24-hour light-dark cycle.
The volunteers stayed during autumn and spring, when the hours of daylight and darkness are roughly the same. Wulff, who is leading the study, found they had significantly higher levels of morning alertness after sleeping in the glass house. Also, the drop in melatonin levels in the morning – marking the end of the biological night – arrived an average 26 minutes earlier, presumably because the dawn light flooded their sleeping quarters.
"Exposure to light at this time is setting up the body to be awake, causing the whole neurochemistry of the brain to be better synchronised," says Wulff. She wants to recruit more people to live in the so-called Photon Space, including during summer and winter.
For those of us who don't live under glass, getting more sunlight in the morning is a good idea. However, it is still unclear how much daylight is necessary to optimise health, and it may well differ depending on what you are trying to achieve. Half an hour in the morning may be enough if you want to stabilise your circadian system, says Figueiro. "But if it's for alertness, you may need bright light exposure for the whole day."
In the West people spend almost all their time indoors, in artificial lightGary John Norman/Getty
Not everyone can walk or cycle to work, sit next to a large window or get outside during their lunch break (see “Optimise your exposure”). But even indoors, we can improve our access to the right light. A national standards body in the UK just released guidelines calling for more daylight in buildings, for instance, and "human-centric" lighting systems are springing up in care homes, hospitals, schools and workplaces. The idea is that by adjusting the blueness – and sometimes the brightness – to more closely replicate conditions outdoors, we could improve our sleep, health and well-being.
In the dementia wing of the Ceres Center, a residential care home in Horsens, Denmark, the lights are an intense white-blue during the day, dimming to soft amber in the evening. Bedrooms are kept dark at night, although residents who get out of bed then will trigger an amber bathroom light. Night-waking is a frequent problem among people with dementia. Not only does this put them at risk of falling, it is also associated with delirium or confusion. A related problem is sundowning, where people with dementia become more agitated, aggressive or confused in the early evening. Both phenomena have been associated with disturbed and flattened circadian rhythms, and they seem to worsen during the short, dark days of autumn and winter, especially when it is cloudy.
A decade ago, a clinical trial in the Netherlands suggested that tweaking the lighting might help. Six care homes were fitted with additional bright lights, raising the indoor illuminance between 10 am and 6 pm to the level you would expect outdoors on an overcast day. In six other homes, the lighting was left unchanged. After 3.5 years, residents who were exposed to the brighter daytime lighting showed less cognitive deterioration and fewer symptoms of depression. When bright light was combined with melatonin supplements, they also slept better and were less agitated.
Bright ideas
Others have tried to replicate these findings, with mixed results. Figueiro suspects that may be down to inconsistencies in the amount of light actually being delivered into each person's eyes. "If people don't take their medication, you're not going to see any effect, but that doesn't mean the medication doesn't work," she says. "It is the same thing with light." To address this, she has been trialling a "light table" that directs light upwards into people's eyes. A soon-to-be-published study will show that this improves sleep, reduces depression and decreases agitation in people with dementia, she says.
Poor lighting is also a problem in many hospitals, which tend to have dim lights left on day and night. A 2017 study found that the average daytime illuminance in a UK intensive care unit was 159 lux. At night, it was 10. People being cared for in such units often have disrupted circadian rhythms, which are associated with impaired recovery from illness, says David Ray at the University of Manchester, UK. Compounding the problem, certain drugs, including morphine, can further disrupt circadian clocks.
Hospital patients recover faster when they have greater access to daylight. In one study, the average length of stay for people recovering from heart surgery was reduced by 7.3 hours for every 100 lux increase in daytime illuminance. Research in mice may provide insight into why this happens. When mice were injured to simulate a heart attack, there was a significant difference in the number and type of immune cells that rallied to the heart, and in the amount of scar tissue that formed, depending on whether they recovered in a cage with a disrupted light environment, as you would find in many hospitals, or a more natural 24-hour, light-dark cycle. Those with disrupted rhythms were more likely to die from their injury.
As for whether human-centric lighting can help those who aren't living in a care home or recovering in hospital, there is some evidence that it can improve healthy people's sleep. But Figueiro is less convinced by other claims, for example that it boosts productivity. "In many cases, human-centric lighting is being oversold and overstated," she says. "Usually, people are only talking about spectrum when it comes to circadian lighting. They are not talking about the amount, timing or distribution, and it is how much light that's getting into the eyes that matters."
There is still much to learn about how light – and darkness – affect our biology. "How much light do we actually need, at what time of day do we need what, and how often do we need it? We just don't know these things at the moment," says Wulff. Yet as a basic principle, we should all be striving to brighten our days and darken our nights. We evolved on a planet with a 24-hour cycle of darkness and light. It is time to reconnect with those extremes.
Optimise your exposure
DAYTIME
Lighting should be bright and blue-white. Daylight is better
Wake up at a regular time and open the curtains as soon as you get up
Eat breakfast in the brightest, sunniest area of your home – or outdoors
Walk or cycle when you can
Change where you usually sit to be next to a window, or consider investing in a very bright desk light
Get outdoors as much as you can, but beware skin damage
Swap indoor exercise for an outdoor equivalent
EVENING
Lighting should be dim and emit a warmer colour
Use table lamps, rather than bright overhead lights
Install warm-toned light bulbs or invest in dimmable, colour-changing bulbs
Listen to your body and go to bed when you start feeling sleepy
Use blackout blinds to block the light from street lamps
Tracking the rays Joshua Howgego
Am I getting enough sunshine? As evidence piles up about the harms of spending so many hours inside in a kind of perpetual dusk, I have started to wonder if I am being exposed to all the light I need.
To find out, I downloaded a phone app that measures light intensity. When I first fired it up, I could see out of the window about 5 metres from my desk in the New Scientist office that it was an overcast afternoon. Yet it felt reasonably light and airy where I was sat, and the app told me I was bathed in 134 lux of light. That sounded all right.
But as I began taking readings at regular intervals — at breakfast, during my commute, at my desk, at lunchtime and at home in the evening — I realised just how much time I was spending inside in relatively dim conditions. When I ate lunch outside on a bench the next day under a mixture of sun and clouds, the lux reading was 4111. I was taken aback at how much higher the number was. It didn't feel like I was sitting in the gloom when I was indoors, but all the readings I took when inside hovered around the 200 to 500 lux mark, so many times lower than on the bench.
With my baseline numbers in, I began to think about how I could get a bit more sunlight into my apparently Gollum-like existence (see “Optimise your exposure”). That basically meant getting outside as much as possible. I got back in the routine of cycling part of the way to work, and although I struggled to measure the light with my app while riding, I assume that helped. I also ate my lunch outside unless my food consisted of leftovers that needed microwaving or it was freezing or raining. Unfortunately, in early spring in London, this turned out to cover most days.
Could technology help? I already avoid screens in the evening, but wondered whether I could get more bright light during the day, even while seated at my desk. For that I borrowed a daylight-simulating light from a firm called Lumie. The plan was to switch it on for 10 minutes each morning to give me a blast of artificial sunshine. Disappointingly, my app only registered about 465 lux, even about 50 centimetres from the light. That was despite the lamp being so blinding, it was hard to work at my computer while it was on.
In the end, what I learned seems obvious: I need to prioritise getting outside more. In the hustle and bustle of daily life, it is surprising just how easy it is to pass day after day in the indoor gloom.
Just how bad is blue light?
You have probably heard that being exposed to blue light in the evening can disrupt your circadian rhythms and imperil your health. But how bright – and how blue? Mariana Figueiro and her colleagues of the Lighting Research Center at Rensselaer Polytechnic Institute in New York state are testing this by measuring melatonin, a hormone linked to sleep that rises during the evening in response to a signal from your body clock.
Several studies have suggested that the light from smartphones can disrupt sleep, but Figueiro's team found that only larger devices produce enough bluish-white light to affect levels of melatonin: for adults, it was 85 lux for 1 hour's exposure, while for adolescents it was 71 lux. "A larger iPad could provide that amount of light, or a little bit more, but phones typically don't," says Figueiro. However, they found that lower light intensities could suppress melatonin if people were exposed to them for longer periods of time.
For light sources emitting a warmer colour, such as incandescent light bulbs or warm-white LEDs, the typical levels found in a living room in the evening wouldn't suppress melatonin production. Nor would a TV screen watched from 2 metres or more.
That may be key: how close the light is to your eyes can make a difference. In a separate study, Figueiro discovered that even in night-shift mode, which reduces light from the blue part of the spectrum, an iPad could suppress melatonin. Her recommendation: "Just turn it off."
Linda Geddes is a New Scientist consultant and author of Chasing the Sun: The new science of sunlight and how it shapes our bodies and minds