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

The cycle of sleep and wakefulness is one of the key human behaviours.

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

When asleep, our brain memorises and processes information. Our body clears toxins and repairs itself, allowing us to function properly when awake.

Even short-term sleep deprivation significantly affects our wellbeing. Most of us begin to fall apart after just one night without sleep and after three nights of missed sleep, we are functioning way below par.

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

These effects worsen over time. The longest documented period without sleep of just more than 11 days prompted serious cognitive and behavioural changes, problems with concentration and short term memory, paranoia and hallucinations.

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

Setting the body clock

The reason light is so important is that it sets our circadian rhythm, or body clock, via specialised light sensors within the eye.

Our eye detects the light and dark cycle within our environment and adjusts the body’s circadian rhythm so that the internal and external day coincide.

This is so powerful that that people who have very severe eye damage can find their body clock is thrown off , leading to sleep problems.

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

Jetlag is the most obvious example of the effect light can have. Exposure to light in the new time zone helps reset our body clock to local time, telling us the right time to sleep.

In 1800, most people across the world worked outside and were exposed to the change from day to night.

Today, many of us miss out on these environmental cues as we work inside. Agriculture and fishing, for example, now make up just 1% of jobs in the UK .

We have become a light deprived species, and this has far reaching consequences for the quality of our sleep, and consequently our wellbeing. The optimum amount varies from person to person, but we do know that our bodies need exposure to very bright light that the majority of indoor lighting does not provide.

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

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

Working the nightshift

While many of us aren’t getting enough natural light, for nightshift workers it is a particular issue.

They have to work at a time when the body clock has prepared the body for sleep, and alertness and performance ability are low. They may try to make up on sleep during the day, but it will usually be shorter and of poorer quality.

In effect, they work when they are sleepy and sleep when they are not, and the negative health effects of this are only just being fully realised.

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

Over the long term, many aspects of health can be affected by nightshift working, which may shorten life spans by up to six years.

As many as 97% of nightshift workers fail to adapt to the demands of their work pattern, regardless of how many years they have been doing the job.

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