Circadian Rhythm – How Your Internal Body Clock Works

image of a clock on the planet earth

Your body is programmed to do certain things, like go to sleep and wake up, at about the same time every day or night. This is known as a circadian rhythm.

If you suffer from occasional sleep problems or have a more serious sleep disorder, it can help to have an understanding of the natural daily rhythm which your body would prefer to follow.

In this article you’ll find out how the circadian rhythm works and how you might be able to harness your knowledge of it to sleep better.

What is a circadian rhythm?

A circadian rhythm is a biological process which repeats itself every 24 hours. It comes from 2 Latin words:

  1. circa = around
  2. diem = day

So a Circadian rhythm takes ‘around a day‘ to happen. This isn’t just something which is unique to humans though – many animals and plants also have circadian rhythms.

Processes which happen at the same time every day

Some of the common elements of the human circadian rhythm are shown in the diagram below. You can see how at the same time every day your body should be doing particular things.

For example, you have the best muscle strength at around 5pm every day. Just in time to get you through rush hour after work, or fuel you for a trip to the gym.

a diagram showing the circadian rhythm timings in a day

There are many factors that influence your sleep pattern. One of the most important is the hormone melatonin, and the time of day your body produces more or less of it.

You can see in the diagram that on average people start producing melatonin at around 9:00 pm, and stop around 7:30 am. This means that if you go to sleep after 9 pm, you might find you have a better chance of falling asleep than going to bed before 9 pm.

And if you get up after 7:30 it could be easier than waking up at the crack of dawn. These times are also close to the natural cycle of daylight and darkness in many countries. And indeed the circadian rhythm is also controlled to a certain extent by the cycle of day and night.

So people (and many animals too) are biologically programmed to prefer sleeping during darkness and to be awake during the day.

Of course we don’t all sleep and wake at the same time and we may have differences in our rhythm. Equally, if you ignore these natural factors, then you may find it very difficult to sleep or wake up.

This set cycle of body functions also explains why some shift workers find their sleep is badly disrupted, especially if the shifts change frequently.

Another example of differences in our rhythm is often seen in teenagers. Research is increasingly showing that teenagers’ body clocks are time shifted later than children and adults.

This can have a significant impact on their ability to get up early and focus on school or college in the mornings. You can read more about this in the article about teenager sleep patterns.

How the circadian rhythm works

We don’t normally talk about our circadian rhythm in everyday conversation. But we do talk about something else which plays a crucial part in controlling the circadian rhythm – our ‘body clock’.

Your internal body clock is really a small part of the brain called the suprachiasmatic nucleus. This is a pair of tiny structures in the brain which you can see in the picture below:

diagram of the diagram of the suprachiasmatic nucleus

How light and the body clock work together

When your eyes see daylight, signals travel from the eyes down the optic nerves and arrive at the suprachiasmatic nucleus. The suprachiasmatic nucleus then sends its own signals to different areas of the brain.

One such area is the pineal gland, which reacts to these signals by stopping the production of melatonin. When the level of melatonin in the body drops, you start to feel more awake.

And the reverse process happens when light starts to fade in the evening. The internal body clock demands that the body produces more melatonin, which then starts to make you feel sleepy.

So light sends signals to the brain, and your internal body clock then switches on or off the hormone which makes you feel sleepy.


The influence of light on your body clock is known as a zeitgeber. Zeitgebers are the external factors that influence your circadian rhythm.

So essentially you have an in-built body clock, but it also depends on outside factors like light to work efficiently.

When your internal body clock is disrupted

airline pilot sitting in a cockpitThe circadian rhythm of sleep is unfortunately quite sensitive and can easily become disrupted.

It’s most commonly disrupted due to changes in how much daylight you get or by the choices you make about your daily schedules.

There are a few situations and sleep disorders which are associated with body clock disruption:

Seasonal Affective Disorder

In the winter months some people find their mood is affected as the daylight hours are reduced. If severe enough, it’s called Seasonal Affective Disorder.

It can affect their mental health by making them feel low in mood along with other symptoms similar to depression.

Delayed Sleep Phase Syndrome

Some people suffer from a sleep disorder known as delayed sleep phase syndrome – this makes them want to go to sleep much later than usual. Teenagers often naturally experience this, resulting in sometimes being called lazy!

Jet lag and shift work

Some people have temporary problems due to jet lag or working unusual and difficult shift patterns. These problems can be difficult to manage in some jobs, for example airline pilots and stewards who frequently change time zones and have to adapt to them.

Use of electronic devices

There’s mounting evidence that the use of electronic devices such as tablets and mobile phones near bedtime can disturb your body clock and impact on your sleep.

How insomniacs can use this knowledge

Spending time in the daylight can help insomniacs keep in tune with their circadian rhythm.Your body naturally wants to help you sleep at night by producing the sleep hormone melatonin, as well as reducing your body temperature.

Understanding your own body clock and trying to help where possible, rather than fight it, might prove beneficial. Some aspects you might want to consider include:

  • It’s a good idea to go to try to go to bed and get up at the same time every day.
  • Eating large meals just before bed can raise your body temperature, countering the natural cycle of reducing temperature just before bed.
  • Although any exercise is good for helping you sleep, perhaps too vigorous a workout just before bed might not be the best answer.
  • You can also use the knowledge of your body clock and follow the cycle of day and night as closely as possible. If you live in a country where there are very long days or nights, then this of course could be difficult. But you can still try to plan to sleep after 9 pm and be awake after 7:30 am.
  • If you work shifts, suffer from jet lag or are traveling, then also remember than you can help your internal body clock to reset. However, it’s not always the best idea to be in daylight when you travel. It depends on which direction you go and how many time zones you cross. To find out exactly how to work this out, take a look at the article about beating jet lag.


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