Light and circadian rhythm: the key to a good night's sleep?

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The cycle of sleep and wakefulness is one of the main human behaviors.

We spend about a third of our lives sleeping and cannot survive without it.

When we sleep, our brain stores and processes information. Our body eliminates toxins and repairs itself, allowing us to function properly when we are awake.

Even short-term sleep deprivation significantly affects our well-being. Most of us start to collapse after only one night without sleep and after three nights of missed sleep, we are working well below par.

One study suggested that after 17-19 hours of stay, performance on cognitive tasks may be similar to having too much to drink.

These effects worsen over time. The longest documented period of no more than 11 days of sleep caused severe cognitive and behavioral changes, problems with concentration and short-term memory, paranoia and hallucinations.

But while scientists have long understood the importance of getting enough sleep, sometimes the key part of light exposure can be overlooked.

Setting the biological clock

The reason that light is so important is that it sets our circadian rhythm, or biological clock, through specialized light sensors inside the eye.

Our eye detects the cycle of light and darkness within our environment and regulates the circadian rhythm of the body so that the internal and external day coincide.

This is so powerful that people who have severe eye damage can find that their biological clock is ejected, leading to sleep problems.

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Without any access to light, the human body clock seems to drift, adding about half an hour to its 24-hour cycle for each day of darkness.

Jetlag is the most obvious example of the effect light can have. The exposure to light in the new time zone helps to bring the biological clock back to the local time, indicating the right time to sleep.

By 1800, most people around the world were working outside and exposed to change from day to night.

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Today, many of us lose these environmental signals as we work inside. Agriculture and fisheries, for example, now constitute only 1% of jobs in the UK.

We have become a species without light and this has far-reaching consequences for the quality of our sleep and, consequently, for our well-being. The optimal quantity varies from person to person, but we know that our body needs exposure to very intense light that most of the interior lighting does not provide.

A notable side effect is Seasonal Affective Disorder (SAD), a form of depression that is thought to affect 2-8% of Europeans, which is linked to the lack of exposure to sunlight.

And there are many other areas where the lack of natural light has caused problems.

Working on the night shift

While many of us do not get enough natural light, it is a particular problem for night workers.

They have to work at a time when the biological clock has prepared the body for sleep, and readiness and performance skills are low. They may try to catch up on sleep during the day, but they will usually be shorter and of lower quality.

In fact, they work when they are sleepy and sleep when they are not, and the negative health effects of this are only fully realized.

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In the short term, it can induce abnormal emotional responses and an inability to process information correctly.

In the long term, many aspects of health can be affected by night work, which can reduce life span up to six years.

About 97% of night workers fail to adapt to the needs of their work model, regardless of how many years they have worked.

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They are not able to change their biology because the artificial light found in an office or a factory is very weak compared to the ambient light; from noon on a sunny day, natural light can be 250 times brighter than office light.

When a night shift worker leaves to go home, they are often exposed to this bright natural light, sending signals to their internal timing system that it is time to wake up.

In a Harvard study, night shift workers became completely nocturnal after being exposed to bright light at work and therefore completely protected from natural light during the day. But this is not a practical solution for most people.

Increased natural light

Even the residents of nursing homes often suffer from lack of sunlight.

Interior light may be poor, while residents often experience very little natural light, which means that poor quality sleep is a common complaint.

A Dutch study increased the light in the common areas of a nursing home, while trying to make the rooms as dark as possible.

This seemed to reduce the daytime nap and stabilize nighttime sleep, improving mental capacity and a sense of well-being.

Light deprivation is not just about the loss of intense natural light; it also concerns the exposure times to light.

The twilight light delays our biological clock, making us get up later the next day. The morning light advances the clock, which makes us get up earlier.

When we worked outside it wasn't a problem; we were exposed at sunrise and sunset and so the two canceled each other out.

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But today many of us experience only part of the sunrise-sunset cycle. This can be especially true for college students, who tend to start the next day, and therefore spend more time outside in prime time.

The twilight light delays their body clocks, which means that they will probably get up and go to bed later. This is aggravated by hormonal changes in adolescence and early adulthood that delay the biological clock by about two hours.

The health consequences of smoking, alcohol and unprotected sex are well publicized, but the importance of sleep and the fundamental role of light are probably less well known.

Further research and awareness in this area could help people make informed choices about the priority of their sleep and sufficient sunlight. It could also influence the way in which policies are developed by governments, educational institutions and workplaces.

Minimizing light exposure before going to bed and trying to get us as much morning light as possible are simple steps that could help most people regulate and improve their sleep.


About this piece

This piece of analysis was commissioned by the BBC by an expert who works for an external organization.

Russell Foster is a colleague of the Royal Society and Academy of Medical Sciences. He is also a professor of circadian neuroscience and head of the Nuffield Laboratory of Ophthalmology, which is part of the University of Oxford.


Edited by Eleanor Lawrie

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