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What is a circadian rhythm?

A circadian rhythm refers to biological processes that occur naturally on just over a 24-hour cycle. Rhythms exist everywhere in biology and while we typically have one sleep-wake cycle per day, our sleep occurs in several repetitive cycles of ~90 minutes, so there is a rhythmicity to these sleep stages. Another example of a repeating rhythm is the menstrual cycle, which of course has an approximate cycle length of ~28 days that repeats.

  • A rhythm lasting less than 24 hours (a sleep cycle), which we call an ultradian rhythm.
  • A rhythm lasting more than 24 hours (the menstrual cycle), which we call an infradian rhythm.

So, as you have guessed by now, a rhythm of about 24 hours is what we define as a circadian rhythm. Here are two examples of biological processes that occur with a circadian rhythmicity:

  • Hormones – such as melatonin, cortisol and leptin
  • Sleep-wake cycle – sleep anticipation in the brain default mode network (DMN).

Circadian biology has many health implications. Working a shift pattern that causes circadian misalignment is classified as a ‘probable carcinogen’ by the World Health Organisation. This is one of the quickest ways to deteriorate our health outside of total sleep deprivation.

Evidence suggests that if the circadian rhythm is experimentally disrupted, metabolic syndrome and obesity, premature ageing, diabetes, cardiac arrhythmias, immune deficiencies, hypertension and abnormal sleep cycles develop.

Therefore, is there more need for flexibility around working hours? The following diagram shows a typical circadian rhythm, with hormone activity, cognitive and physical considerations.

Graphic 1

Looking at this diagram, are there considerations around meeting times, when you might exercise or how you structure your evenings?

The science of sleep

Our need for sleep starts from the moment we wake up. Our sleep need rises throughout the day and leads to the secretion of melatonin. Melatonin readies the body for sleep and responds to light. Our environment is full of synthetic light which is very different to our primal ancestors.

As you will see from the following diagram, adenosine is another key hormone in the sleep process. Adenosine builds up in our body during the day as a by-product of using our energy stores. It binds to adenosine receptors in the brain and as levels rise there is a corresponding rise in sleep need. The longer you remain awake, the greater your adenosine levels become, increasing your need to sleep with the need eventually getting to a critical point. When we are asleep, adenosine levels decrease as the chemical is broken down. If you do not get enough sleep, some excess adenosine levels may remain causing feelings of grogginess.

Graphic 2

You will also see that cortisol spikes in the morning. This is an evolutionary response that readies us for stress, helps maintain blood sugar levels, keeps inflammation down and controls the sleep-wake cycle. A great way to regulate cortisol levels is exercise, self-care and good quality sleep.

Which other hormones are pivotal to sleep?

Growth hormone

  • Essential for growth and tissue repair
  • Produced in the pituitary gland (in the brain)
  • Released during sleep

Antidiuretic hormone (ADH)

  • Prevents the production of dilute urine
  • Produced in the pituitary gland (in the brain)
  • Levels of ADH increase during sleep


  • Signals to the body that it is time to sleep
  • Produced in the pineal gland (in the brain)
  • Released with increased darkness


  • Involved in childbirth, lactation and social behaviour
  • Produced in the hypothalamus (base of the brain)
  • Levels peak after five hours of sleep
  • Levels may influence the content of dreams


  • Involved in over 300 functions including lactation, metabolism and immune system regulation
  • Produced in the pituitary gland (in the brain)
  • Levels are higher during sleep than in the day time

So, what is a sleep cycle?

A sleep cycle lasts about 90 minutes, and during that time we move through four stages of sleep. The first three stages make up our non-rapid eye movement (NREM) sleep, and the fourth stage is when rapid eye movement (REM) sleep occurs.

Sleep cycle

NREM sleep

Across these three stages we move from very light sleep during stage one down to very deep sleep in stage three. It is very difficult to wake someone who is in stage three sleep. Across NREM sleep, we have little muscle activity and our eyes do not typically move, but all of our muscles retain their ability to function.

REM sleep

As the name would imply, during this final stage of sleep we have bursts of rapid eye movement. This is the stage of sleep in which most dreaming occurs.


We’ve traditionally been advised to get eight hours of sleep, however we would advise you to consider how many cycles you are getting. Aim for four to six cycles of sleep per night. That feeling of grogginess can occur when you wake in the middle of a cycle, so consider your timings.

If you know your alarm is sounding at 6am, attempt to go to sleep at 10.10pm or 11.40pm (allowing 20 minutes to fall to sleep). This means that you’ll wake at the end of cycle four or five, when you’re out of deep sleep.

Effects on our health

Implications of poor sleep:

  • Memory issues
  • Mood swings and struggling to cope with change
  • Poor concentration and lack of creativity
  • High blood pressure
  • Drowsiness
  • Weakened immune system
  • Weight gain
  • Risk of serious illness, such as heart disease and diabetes.

Benefits of good sleep:

  • Helps reduce stress
  • Can improve your memory
  • Lowers your blood pressure
  • Helps your body to fight back against injury and illness
  • Improves mood
  • Helps you maintain a healthy weight.

There are many implications of good and poor sleep. One to highlight with the changes and potential stress we are facing at the moment is productivity and mood. We are all dealing with some sort of change and our ability to deal with this is hugely impacted by our quality of sleep.

What can we do?

Here are a few facts and ideas to improve your sleep quality:

  • You can’t bank sleep. Don’t believe you can get to the weekend, sleep 12 hours both nights and all will be ok.
  • Sleep happens when you are calm and ready, so do everything you can to achieve this.
  • 20-minute naps will not affect you’re your need for sleep but napping for too long will.
  • Be positive about sleep. We can easily talk ourselves into a bad night’s sleep so be optimistic about it.
  • Control the environment. Invest in good pillows, bed linen and blackout curtains.
  • Nail your routines. Consider what you’re doing before and after sleep.
  • Plan your bedtime to ensure you can wake at the end of a cycle.

Top tip – digital downtime

Put your phone away at least 90 minutes before your bed time. Allow yourself some time to detox from blue light, information and social media. Instead, use this time to process the day, read, perform some stretches or spend some quality time with your loved ones.

Download stretching exercises