Show me the light! The importance of light in regulating your body clock and controlling your sleep-wake cycle.

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Have you admired a nice chimney recently? I imagine not. You may be missing out. Not getting enough daylight is one of the primary causes of poor sleep. Light is absolutely fundamental to regulating our body clock and so to the sleep-wake cycle. The pressures of modern life often see us dashing out to work, spending all day in an office, lunch at your desk and then dashing home barely getting more than 10 minutes of continuous daylight. So, could more light help us sleep better?

Your brain has evolved to use natural light to keep your body clock and sleep-wake cycle on track. 10 minutes here and there is not enough.

In the 1970’s researchers discovered that the body clock, or the circadian rhythm, is located in the suprachiasmatic nucleus (SCN). This small cluster of cells regulate your biological states, from your hunger to your sleep-wake cycle. Experiments have shown if the SCN is damaged the sleep-wake cycle will collapse.

The SCN is located just below the nerves which take information from the eyes to the visual centres of the brain. In fact, the retina of the eye actually connects directly to the SCN which helps explain why light has such an influence on your body clock. This is explains why almost 90% of blind people have disturbed circadian rhythms.

How does it do this? In the middle of your brain between the two hemispheres lies your pineal gland which produces a hormone called melatonin. Melatonin is produced in a fairly predictable daily rhythm and levels start to climb late in the evening and start to decrease after dawn. Melatonin induces drowsiness and sleep and strongly influences your circadian rhythm. Your pineal gland produces melatonin in response to light, or more accurately, reduces the levels of melatonin (and so sleepiness) in response to increasing levels of light.

Failure to expose yourself to natural light and a stable light / dark cycle will result in your circadian rhythm drifting and your sleep-wake cycle being disrupted. Put simply, not having enough light will stop you from sleeping better –  you will find it harder to feel tired, fall asleep and you’ll not wake up feeling alert.

Are we getting enough light?

It is essential you get enough light, especially in the morning. This prompts your pineal gland to reduce the production of melatonin which will help shake off your drowsiness. The more light you get during the day the better – by keeping your melatonin levels low you are in-effect reminding your brain it’s day time and time to be alert. In the evening, by reducing the light you are exposed to, your melatonin levels will begin to rise inducing drowsiness and eventually sleep.

Research carried out by the Research Centre at Rensselaer Polytechnic Institute, New York, showed that those exposed to increased light levels between 8am – noon fall asleep more quickly and have fewer sleep disturbances than those exposed to lower light levels.  Similar results were published in 2017 in the Journal of The National Sleep Foundation.

Before we had electric lights our melatonin levels and our circadian rhythms were controlled by natural light. This is how we’ve been for 150,000 years. And our light detection system is old. Recent research has shown the light detection system in the eye is independent and functionally very different to the visual system. It’s believed this light detection system evolved before the visual system, such is the importance of having and regulating daily rhythms.

Humans have evolved under bright light conditions. Even on a cloudy day in England natural light is about 10,000 lux (lux is the measure of illumination over an area). On a bright sunny day (when we have them) natural light reaches 100,000 lux. Yet we live and work in houses and offices that are cut off from natural light. Artificial light in rooms is usually between 200 – 400 lux. The Chartered Institute of Building Service Engineers recommends various light levels for a range of locations and properties and recommends just 500 lux in offices. This is 20 times less light than a cloudy day in England. We are living in permanent dusk.

Our life styles and lack of natural light are negatively affecting our sleep-wake cycle.

Is all the light the same?

Blue light is the most effective wavelength at influencing your pineal gland, circadian rhythm and sleep-wake cycle. This has been found to be true in all studies undertaken to-date on this subject. And blue light is everywhere. Blue wavelengths are short and high energy and when they collide with our atmosphere they scatter blue light everywhere – that’s why our sky is blue. That’s probably why we have evolved to use blue light to set our circadian rhythm.

So if you want to reduce sleepiness and stay awake and alert, exposing yourself to blue light is the way to go. Here’s the bad news. If you want to begin to feel drowsy in the evening and be ready to sleep at bed time, you need to avoid blue light. However, artificial sources of blue light are everywhere, phones, computers, televisions and energy-efficient fluorescent bulbs.

Technology and electronic devices have, in recent years, begun to use LED backlight technology which enhances screen brightness and clarity. This emits very strong blue light waves. So the more screen time you have in the evening the more you are waking yourself up and increasing alertness and cognitive functioning – absolutely the opposite of what you want to be doing before bed.

It doesn’t take much to interrupt your sleep-wake cycle. It’s now known relatively little light, as little as 200 lux, can suppress night time melatonin production. A phone emits about 50 lux which is about half that of a room light. We know that this 50 lux has a cumulative impact and over the course of a week, exposure to this will affect the sleep-wake cycle and the circadian rhythm. Using screens all evening every evening will start to seriously affect your sleep-wake cycle.

So two things to think about.

  1. Make sure you get enough natural day light in the daytime. This is particularly important in the morning. Go for a walk, get out of the office, and use your lunch time to get natural light. A great tip is to look up to ensure you’re getting maximum exposure to natural light. A great way to do this? Look up and admire the chimneys!
  2. Stop using screens and watching TV a few hours before you go to bed. Rule of thumb for me is at least 2 hours before you go to bed or two and half hours before you want to be asleep.

Written by
Sam Billington

Sam is our sleep guru. I am fascinated by sleep and I've used my science training to research and learn about sleep. I have been providing sleep advice and personal sleep plans for years. My day job is Head of Workplace Culture in Defra, I live in London with my wife and have a PhD in Ecology and Biology.