Your Circadian Clock Your body clock and light. The two most important things that affect your sleep.

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We’ve written about the drive for sleep and how understanding this can help you sleep better – find that here.  How sometimes your sleep debt can compete with your body clock. How sleeping better means getting these drives into sync.  

Your body clock and how and why your wake-sleep cycle has evolved is so crucial to sleeping better, it’s worth a detailed look.

What is a body clock?

Two scientists, Curt Ritcher and Erwin Bunning, developed the concept of an internal clock back in the 1920 and 30s.  The concept of the internal or body clock has been key in biological research ever since. Understanding and using it is key to sleeping better.

Your body is great at keeping your internal environment constant despite changes around you.  For example, your body will work hard to keep your temperature the same in changing weather or after a bout of exercise. This regulation is called homeostasis and makes sure everything from your oxygen levels to hydration all stay about the same.

The body clock, or circadian rhythm, also controls your internal environment.  It does this based on predictable changes to the external environment, such as day and night.  The circadian rhythm provides a predictive 24 hour baseline for your body. It can be supported or overridden by your homeostiatic drive to accommodate sudden physiological challenges.

The circadian system consists of a 24 hour pacemaker or ‘clock’.   This clock is set or reinforced by the drum beat of natural signals we see and feel all around us.  The most obvious signal is the light-dark cycle.

Why bother having a body clock?

Life is difficult.  Not in a ‘I don’t like my boss’ way, but life is in essence an arms race to survive.  Before we escaped from the teeth and claws of nature we were like every other creature. We hunted for our food, we were hunted as food, we fought with and killed competitors.  Life was pretty tough. Any advantage we could get, the better the chance we’d survive.

The 24 hour light-dark cycle is predictable and results in profound changes to the environment.  Light, temperature, predation risk, food availability to mention a few, all change during the day.  Any organism that can tell the time and predict these changes will have a huge advantage.

Humans evolved about 200,000 years ago.  That’s 73 million days. That’s a lot of sunrises and sunsets.  Our ancestors were around for a lot longer than that. In fact, life has been around for over a trillion sunrises and sunsets.  We’ve had a lot of time to evolve an internal clock so we’re able to ‘know’ the time of day. This knowing generates daily bodily rhythms giving you the upper hand in the arms race of life.

You don’t have to wait until it’s morning to be ready to start the day – you’re ready.  You don’t have to wait until it’s dark to be ready to sleep – you’re tired. You don’t have to wait until you see a lion hunting in the evening to be ready to run – your body knows when to vigilant. During the day time when we’re active and looking for food, our organs are ready to take up, process and use nutrients. During sleep the organs slow down but many other essential activities occur such as removing toxins from the brain or reinforcing learning.  The timing of all these processes is controlled by your circadian rhythm.

The circadian rhythm also stops everything happening at once so you can cycle through biological process in the right sequence. The getting of energy (food), digesting it, breaking it down, getting it to your muscles, absorbing it and then using it to repair cellular damage needs to be done in an order.  This is controlled by your circadian rhythm.

In short, your body clock makes efficient use of time.  This gives such a huge advantage that every organism alive today has evolved some kind of body clock.

Where is it from and who keeps the time?

Your circadian rhythm is in your genes and you are primed to run on a 24 hour clock.  That’s what 200,000 years of evolution will do. Your circadian rhythm involves changes in gene expression, protein modification and finally changes in behavior.  

This rhythm is controlled at the level of your genes but they need help to set the correct time.  Your clock must be synchronised to your local environment. This is done by tuning in to the rhythmic changes in the environment, called zeitgebers.

The light and dark cycle is the most important zeitgeber and is often described as the primary zeitgeber.  There are many other other zeitgebers including temperature, rainfall and food availability. The syncing of your 24 hour clock with rhythmical environmental patterns is called entrainment.

Circadian Rhythm and sleep

Sleep probably evolved in response to a 24 hour world where light, temperature, food and how other organism behave all change. Sleep avoids movement in an environment we are poorly adapted to and gives us the time to undertake important housekeeping of the brain and body.

The drive for sleep is regulated by a combination of the sleep debt or homeostatic drive and the circadian rhythm.  From the second you wake you begin building up a need or pressure to sleep. The sleep pressure dissipates when you are asleep. The circadian rhythm timestamps your brain with the most biologically appropriate time for wake-sleep.

If everything’s in sync, your circadian rhythm will tell you it’s bed time just as the pressure to sleep makes you too tired to stay awake. The opposite should happen in the morning, you’ve paid off the sleep pressure as the circadian rhythm kicks in to drive wakefulness.

Most circadian clocks use light as the primary entrainment signal.  Circadian clocks are never bang on 24 hours. To prevent circadian drift and a body clock out of whack with the local environment, light acts to reset the clock every day.

Keep an animal (including a human) in complete darkness and the circadian rhythm will free run and the wake-sleep cycle will break. If you show that animal bursts of light, the effect on its circadian rhythm and sleep will depend on what time the circadian clock thinks it is. If the animal is shown light when the circadian clock thinks its day time there will be almost no effect on the free-running clock.  If you show a burst of light when the circadian clock thinks it’s night time the circadian rhythm will be affected.

So the circadian rhythm’s sensitivity to light is time dependent.  If, in the example above (poor animal, kept in dark room), the animal is shown light in the first half of what the animals circadian clock thinks is night, it’s activity will be delayed the next day. If the animal is exposed to light in the second half of its subjective night, its activity the next day will be advanced.

Thankfully we’re past putting animals in dark rooms to work out what’s going on.  Light exposure is critical to the setting and timing of your circadian rhythm. Light exposure at dawn and dusk is the most powerful.  Getting light at dawn and dusk pulls or pushes your 24 hour body clock to the 24 hours of day-night. Get light at the start of the day and it will make you feel tired earlier in the evening.  Get light in the evening and it will make you feel tired later the following evening. This is the key to maintaining your body clock.

The circadian rhythm does a great job of averaging dawn and dusk times to take into account seasonal variation.  Melatonin plays a role in informing the brain what season it is.

Eye See

So, how does light do this?  Cells sensitive to light, photoreceptor cells, recognise and respond to light.  Invertebrates, fish, reptiles and amphibians have these cells in their brains. Light is able to directly penetrate their brains to activate these cells.  Some, such as fish, have photoreceptor cells on their bodys. These creatures can sense light-dark even if they lose their eyes.

We mammals only have photoreceptor cells in our eyes. These are called photosensitive retinal ganglion cells or pRGCs.  A tiny amount exist in mammalian eyes, accounting for just 1 – 2 % of retinal ganglion cells.

These pRGCs use blue light sensitive photopigment (this is why blue light from tablets, phones and T.V.s is terrible for sleep) to activate. The signals from these cells run into the brain via the optic nerves and to a part of the brain called the suprachiasmatic nucleus (SCN).  The SCN is a pair of small nuclei that sit at the base of the hypothalamus.

The SCN is your circadian clock.  The SCN is made up of several regions or clusters of cells. These are made up of autonomous single cell circadian oscillators that generate a stable rhythm or beat of firing neurons.  This firing is the ticking hands of your body clock. Ticking hands set by blue light entering your eyes.

The Great Circadian Disruption

We live in a 24/7 world.  Artificial light, jet lag, friends and colleagues in different time zones, working indoors, early get ups and late nights.  We are far removed from the light-dark cycle we evolved to use to set and maintain our 24 hour circadian rhythm. Like never winding a watch or the battery running low on a clock, our body clocks are drifting further and further from the natural 24 hour day-night cycle. This has been called ‘The Great Circadian Disruption’ and is having a significant effect on our sleep and health.  

What to do

Your quality of sleep is heavily influenced by the strength of wake-sleep cycle.  Your wake-sleep cycle is controlled by your circadian rhythm. Your circadian rhythm is controlled by light and is most receptive to being set at dawn and dusk.  Try to get maximum daylight exposure between 06:00 – 09:00 and 17:00 – 20:00. Choose times that best suit your natural wake-sleep cycle and getting eight hours sleep. Simple!

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.