Fall asleep in front of the TV at night? Struggle to get back to sleep after going to the loo? Want to get up earlier but can’t manage it?

The solution may be light, as revealed in this unique experiment, where six people wore light sensors to monitor themselves over a 24-hour period.

Our exposure to light is the latest frontier in health, with growing evidence to show that too much at the wrong time – including when we’re asleep – and lack of the right kind, is linked to an array of health problems, including cancer.

The key is our body clock. Every cell in our body has a roughly 24-hour clock inside it and functions best with a regular day/night routine, explains Anne Skeldon, a professor of mathematics at the University of Surrey and a specialist in light’s impact on sleep.

These cells are managed by a master clock in the brain, which needs strong external signals – i.e. light – every day to stay in sync.

‘Our body clock controls when we digest food most efficiently, when we learn new information and when repair mechanisms are most active – and it tells us when we’re ready to sleep,’ says Professor Skeldon.

So if you’re not getting the right balance of light, it can disrupt your sleep.

In the short term, this can lead to poor decision making and irritability. You’re also more prone to infections.

‘Poor sleep alters gene expression in our immune and stress responses,’ explains Professor Skeldon.

Longer term, a misaligned body clock is linked to obesity, type 2 diabetes, heart disease and other conditions.

Essentially, as light hits the back of the eye, it is interpreted by light-sensitive cells, including intrinsically photosensitive retinal ganglion (IPRG) cells, which are particularly sensitive to blue light.

The IPRG cells combine the information and send signals to the master clock. The role of the IPRG cells is why blue light (e.g. from phones and computer screens) can have more of an impact on the body clock, explains Professor Skeldon.

Bright light exposure (more than 1,000 photopic lux – the conventional unit of light intensity) shortly after waking helps set the body clock, says Stuart Peirson, a professor of circadian neuroscience at the University of Oxford.

(Note, we refer to photopic lux but some apps and research studies measure melanopic lux, which is different – but roughly half the value of photopic lux used here.)

‘It’s hard to get more than 1,000 lux without going outside – daylight is virtually always above this level, even on a dull winter’s day – so a morning walk or even sitting by a window would be enough,’ says Professor Peirson.

Indeed, even in the UK winter, when we feel starved of sunlight, we’re still getting very bright light. On an overcast December day, it can peak at 20,000 lux (in summer, it’s 100,000).

‘And we stop being sensitive to light at 5,000 lux – our visual responses are then saturated in terms of the impact on our body clock,’ says Professor Skeldon.

(Note: Bright light is different from the invisible UV light needed to make vitamin D, and that is low in the UK winter.)

Natural light doesn’t seem to have an advantage over artificial light in terms of our response, especially when it’s the same brightness, says Timothy Brown, a professor of neuroscience at the University of Manchester.

But because natural daylight is much brighter, it may be better for your body clock because then it really knows it is daytime.

Just as important is a strong contrast with evening light.

‘Having too much light in the evening is a bit like coffee – it has an alerting effect and will delay when you feel sleepy,’ says Professor Peirson.

It shifts the body clock later, especially if it is in the three hours before sleep – ‘this will reduce production of the [sleep] hormone melatonin, which can impair sleep and make it harder to wake in the morning,’ explains Professor Brown.

‘Research has shown that increased light at night is associated with a greater risk of conditions such as type 2 diabetes and neuropsychiatric disorders such as depression.’

Research he led, published in the journal PLoS Biology in 2022, showed that ‘in the three hours before bed the maximum level of light – artificial or natural – you should be exposed to is around 20 lux (the equivalent of a dim lamp; a mobile phone can be ten to 100 lux).

‘Where you sleep should be roughly 2 lux maximum – and, when possible, ensure you’re in an environment that’s consistently above the equivalent of 500 lux in daytime, although you’d need to be outside all day to achieve that,’ adds Professor Brown.

But most of us won’t get that: for example, a 2023 study in the journal Proceedings of the National Academy of Sciences found that people going about their everyday lives were exposed to just six hours above this amount a day.

Any outside light is better than nothing, says Professor Peirson.

‘Even brief periods, five minutes at a time, is enough to shift your body clock – and the brighter, the better. So a short walk or just sitting by a sunny window helps.’

Even while sleeping, light can potentially have a negative impact.

A street-lamp outside a bedroom window can be disruptive – low-energy LED street-lights, which give off high levels of blue light – are associated with higher rates of conditions such as depression and type 2 diabetes.

A 2018 study by the University of Exeter and the Barcelona Institute for Global Health found that residents in Madrid and Barcelona with high exposure to outdoor blue light had double the risk of prostate cancer and a 1.5 greater risk of breast cancer.

‘You don’t need blackout blinds – thicker curtains or liners may be just as effective,’ says Professor Peirson. Similarly, low-level lighting in the bathroom is a good idea.

To find out how much light exposure we might be getting, Good Health teamed up with the University of Surrey to analyse the levels people were typically exposed to over a 24-hour period.

Here’s what we discovered – and what it could mean for you…

LIGHT BY WINDOW

David Hodgson, 63, a retired construction specialist, lives in Cumbria with wife Suzanne, 53, a writer.

HIGHEST READING: 47,329 lux (at 11.35am when out driving)

500 LUX OR OVER IN THE DAY: 1 hour

AVERAGE LIGHT EXPOSURE THREE HOURS BEFORE BED: 6.9 lux

AVERAGE LIGHT EXPOSURE THREE HOURS AFTER WAKING: 237 lux

AVERAGE NIGHT-TIME LIGHT: 0 lux (highest reading was 2 lux while his wife was using devices in bed)

DAVID SAYS: I am outside a lot in the day – I walk my dog, Sparky, up to four times, usually at least an hour or two each time.

Our house isn’t very bright – we tend to use lamps – but we do get the street-lamp glow through our bedroom blinds – and if I get up to use the loo in the night, this light means it can take up to two hours to get back to sleep. This happens roughly four nights a week and leaves me grumpy and tired.

My wife tends to read or use her tablet in bed for an hour or so after I go to sleep, and I have to cover my head with the duvet.

EXPERT VERDICT: Despite all that walking the dog, two walks were after the sun went down (4.15pm and 7.50pm) hence David’s relatively low daytime levels.

A street-lamp isn’t as disruptive as you might think – David’s sensor detected barely any light, says Professor Skeldon.

‘Once your eyes are shut, only 5 per cent of light enters through your eyelids, though some people are more sensitive to light.’

Significantly, even though David didn’t get much bright light in the day, he had a strong contrast between daytime and evening light – and that sends a strong message to the body clock, says Professor Skeldon, which contributes to David sleeping well (10pm until 7.45am) both nights.

COMMUTES IN DARK

Nickie Hawke, 40, a civil servant, lives in a large town in South Wales.

HIGHEST READING: 486 lux (at 9.21am when driving to work)

500 LUX LIGHT IN DAY: 0 hours

THREE HOURS BEFORE BED: 8.2 lux

THREE HOURS AFTER WAKING: 56.8 lux

NIGHT-TIME LIGHT: 4.5 lux (highest reading was 71 lux between 1.30am to 3.15am, asleep on sofa)

NICKIE SAYS: In the winter I leave for work and return home when it’s dark – and my office desk is far from the windows (I eat lunch at my desk, too).

In recent years I’ve noticed that I’m not as active in October when the clocks change – I also don’t sleep well and feel depressed. It’s as if my life is on pause in winter.

I usually avoid going to bed until I can’t stay awake and procrastinate by watching TV – I often fall asleep on the sofa or while scrolling on my phone.

EXPERT VERDICT: Nickie didn’t get any bright light in the day, so it wasn’t surprising she has a bedtime procrastination problem, says Professor Skeldon. ‘Even though her low evening light levels are good, there’s little contrast with daytime light.’

The fact she also fell asleep on the sofa from 6pm to 8pm shows there’s insufficient light for her to have a robust daily rhythm with a regular sleep pattern.

Getting so little light can impact mental health.

As well as spending time outside, using a 10,000 lux lightbox for 30 minutes in the morning may help, says Professor Peirson.

Professor Skeldon adds: ‘It might not be possible for Nickie to sit near a window at work, so I’d suggest going outside during breaks, turning up the brightness on her computer and asking for a desk lamp.’

When working from home, she should sit by a window and make sure her home lighting is bright.

USES PHONE AT NIGHT

Sophia Husbands, 46, a careers coach, lives in Windsor.

HIGHEST READING: 31,662 lux (at 10.16am working from home)

500 LUX LIGHT IN DAY: 2.5 hours

THREE HOURS BEFORE BED: 41 lux

THREE HOURS AFTER WAKING: 226 lux

NIGHT-TIME LIGHT: 1.6 lux (highest reading was 412lux at 1.30am when going to bathroom)

SOPHIA SAYS: I tend to turn off my ceiling light at 7pm and only have my lamp on, so I’m surprised my light exposure before bed was high – though I do use my phone late.

I sleep fairly well but I’d like to fall asleep earlier. After I went to the bathroom at 1.30am, it took over an hour to get back to sleep. I work by a window at home so it’s reassuring to know my light exposure is quite high in the day. And I walk for at least an hour every day, at different times.

EXPERT VERDICT: Bedtime procrastination or struggling to get to sleep can be caused by not getting enough bright light in the day, and/or too much light in the hours before bed – and Sophia ‘has relatively high levels of the light in the evening’, says Professor Skeldon.

To stay under the recommended 20 lux would probably mean turning off main ceiling lights and using lamps or dimming lights.

‘Phones don’t produce as much light as you’d think – although I recommend using the “night” setting to filter out some of the blue light, and holding it away from your face,’ says Professor Skeldon. ‘Her night-time high reading was typical: Bathroom lights are often very bright and can wake you up.’

Try to let your eyes adjust to the dark, or use a movement-sensitive plug-in night light, says Professor Peirson. ‘These usually give out less than 10 lux, enough to see by, without affecting the body clock.’

To go to bed earlier, go for a walk earlier in the day.

‘The general rule is light soon after your natural wake time will help you get up earlier the next day, and gradually shift your wake time,’ says Professor Skeldon.

HAS AN EVENING BATH

Carolyn Pearson, 61, a travel security expert, lives in Leeds with partner Sandy, 50, a vet.

HIGHEST READING: 49,193 lux (at 10.38am walking outside)

500 LUX LIGHT IN DAY: 1.2 hours

THREE HOURS BEFORE BED: 36 lux

THREE HOURS AFTER WAKING: 94 lux

NIGHT-TIME LIGHT: 0 lux

CAROLYN SAYS: I work from home under a spotlight in the kitchen and, although I walk once or twice a day, I was surprised to see how little bright light I had and how much I got before bed.

I dim the lights and watch TV, have a bath, then read in bed for 15 minutes with my bedside lamp and a dull ceiling light on – so will use just one now, and in the bathroom, swap from the bright light to a candle and the mirror light.

Generally I sleep well – if I wake in the night, I use an eye mask to block out light. It also helps my mind quieten down.

EXPERT VERDICT: ‘Carolyn’s peak reading is very high, but brief high readings are not as significant as they may seem because our visual receptors saturate around 5,000 lux,’ says Professor Skeldon.

‘Carolyn isn’t getting enough bright light in the day, although she had two walks during the day – so should make her home brighter. It’s also better to have a longer walk shortly after waking for the strongest impact on the body clock.’

Her evening light exposure was 100 lux – this would be fine provided there’s a contrast between bright light in the day and dim light in the evening, adds Professor Skeldon.

‘While her daytime readings aren’t ideal, Carolyn slept well so her clock seems to cope with suboptimal light exposure.

‘Some people find lights from alarm clocks and around blinds quite disruptive – an eye mask is likely to help, even if the light is actually very low.’

WALKS DOGS ALL DAY

Zita Wells, 56, runs a pet care company in Hartfield, East Sussex.

HIGHEST READING: 11,551 lux (at 11.43am while walking dogs)

500 LUX IN DAY: 5 hours

THREE HOURS BEFORE BED: 43 lux

THREE HOURS AFTER WAKING: 95 lux

NIGHT-TIME LIGHT: 0 lux

ZITA SAYS: Some rooms in my house are dimly lit with just lamps, though my kitchen and bathroom are very bright, which explains why my evening light exposure was high overall.

I’m perimenopausal and my sleep has been appalling for two years. I can fall asleep quickly but I wake throughout the night and then struggle to get back to sleep.

But I also burn the candle at both ends so I don’t go to bed early enough (usually around 11pm) – then I’m up from dawn to dusk walking dogs.

EXPERT VERDICT: While Zita’s light exposure in the evening is high, it’s compensated by the many hours of bright light she’s exposed to by day, says Professor Skeldon.

Sleep can be disrupted for many reasons, including the menopause, ‘and as we age, the lens of our eyes and our pupils change, so we may become less sensitive to light’, says Professor Peirson.

‘This will reduce the disruptive effects of light at night, but also reduces the beneficial effects of daytime light – so we need regular bright daytime light to retain robust circadian rhythms.’

WORKS NIGHT SHIFTS

Liam Cooper, 30, a racing car quality inspector, lives in Bedfordshire with fiancee Jodie, 30, a mental health practitioner.

HIGHEST READING: 66,601 lux (at 1.12pm while fishing)

500 LUX IN DAY: 10.1 hours

THREE HOURS BEFORE BED: 314 lux

THREE HOURS AFTER WAKING: 4,832 lux

NIGHT-TIME LIGHT: 599 lux (highest reading was 1,387 lux at 5.13am at work)

LIAM SAYS: I work night shifts, doing five nights from around 7pm until 5.30am, followed by two days off, when I try to readjust to normal sleep patterns to spend time with Jodie.

It’s overbearingly bright at work, with a fluorescent light – and while I fall asleep straight away at home, after about five hours that’s it.

We have normal blinds in our bedroom but bright light still comes in, so that probably doesn’t help.

EXPERT VERDICT: ‘Liam’s working environment is around 1,000 lux and his bedroom is too light [around 80 lux] – getting so much light day and night is disruptive to the body clock,’ says Professor Skeldon. ‘Using blackout blinds or an eye mask could help, and sticking to the same bedtime even on days off.’

Professor Peirson adds: ‘Even after decades of shift work, your body still thinks it’s day when it’s day because just a brief exposure to daylight, such as when driving home, signals this.

‘While long-term studies of shift workers have shown they’re at greater risk of disorders such as cardiovascular disease, this may be related to poor sleep and other factors such as diet: if you work shifts and get enough sleep, this is likely to be a good sign.’