Circadian Rhythms: A Straightforward Guide

Forget your phone, your watch, and your alarm. You already have a clock – a biological timepiece that turns over every 24 hours.

That clock is why you feel sleepy at certain times and hungry at others. It’s why you swing between energy and fatigue, alertness and distraction, relaxation and irritation. It’s the same invisible cycle that shapes the lives of everything from you, your friends, and your family to fruit flies, plants, and bacteria.

We’re talking about your circadian rhythm. In this science-backed guide, we’ll unpack exactly what a circadian rhythm is, what happens when it’s misaligned, and, most importantly, how you can calculate and reset yours.

What Is a Circadian Rhythm?

Every human has an internal clock: their circadian rhythm. That rhythm (known as your ‘tau’) naturally turns over once every 24.18 hours or so [1]. It controls things like when you feel hungry, sleepy and energised.

Even if you were locked in a pitch-black room for days, your circadian rhythm means you’d keep feeling alert, hungry and sleepy at roughly the same times. But, because your rhythm’s length is slightly longer than 24 hours, you would gradually become ‘de-synced’ with the outside world.

Luckily, nature developed a solution to help keep our rhythms on track. Certain external factors, known as ‘zeitgebers’ (German for ‘time-givers’), help entrain our bodies and keep them aligned with the day–night cycle. Those zeitgebers include:

• light
• temperature
• meal timing
• movement
• social activity.

Light is the most important zeitgeber our bodies have, which is why artificial lighting – like phone screens – can seriously affect our sleep.

The Science Behind Circadian Rhythms

If you’re interested in the science behind circadian rhythmicity, this section is for you.

Most cells in your body have their own time-keeping mechanisms – little molecular clocks. These ‘biological oscillators’ can be synchronised or ‘entrained’ by other cells or factors outside your body [2]. For example, the cells in your heart entrain each other so that they oscillate at the same time, which leads to your heart beating [2]. Think of them as billions of little clocks all chiming together to produce one big boom.

Because your circadian rhythm is so complex and involves so many different parts of your body, it needs to be directed by a master timekeeper – a conductor that says ‘go now!’. That timekeeper is your suprachiasmatic nucleus (SCN), a collection of around 10,000 neurons located in the middle of your brain [1].

Your SCN can be entrained by external factors (the zeitgebers we talked about earlier). When certain specialised cells in your eyes detect light, they pass that information to your SCN, which can then use that information to keep all your organs synchronised [1]. For example, when it becomes dark, your SCN tells your pineal gland to release melatonin – the hormone that puts you to sleep.

What Are Chronotypes?

Not everyone’s circadian rhythms run on the same time. Some people – known as Lions, morning larks, or M-types – have advanced sleep phases, which means they’re genetically predisposed to waking up earlier [3]. Others – known as Wolves, night owls, E-types – have delayed sleep phases, which means they are genetically predisposed to waking up later [3]. Lions typically wake up and go to bed 2 to 3 hours before Wolves [4].

Around 52% of the population falls into a third category: Bears or N-types, who don’t have a strong preference for either early or late rising. Together, those 3 categories are known as ‘chronotypes’.

It’s not clear why we’re programmed to wake and sleep at different times. Here’s what leading sleep scientist Dr Matthew Walker thinks [5].

Humans likely evolved to co-sleep as families or even whole tribes, not alone or as couples. Appreciating this evolutionary context, the benefits of such genetically programmed variation in sleep/wake timing preferences can be understood.

The night owls in the group would not be going to sleep until one or two a.m., and not waking until nine or ten a.m. The morning larks, on the other hand, would have retired for the night at nine p.m. and woken at five a.m. Consequently, the group as a whole is only collectively vulnerable (i.e., every person asleep) for just four hours rather than eight hours, despite everyone still getting the chance for eight hours of sleep.

That’s potentially a 50 percent increase in survival fitness.

Example Circadian Rhythm

Here’s an example of a Bear’s circadian rhythm.

What Hormones Are Affected By Your Circadian Rhythm?

Many of your most important hormones are affected by your circadian rhythm. They include [6, 7]:

• melatonin (the sleep hormone)
• cortisol (the stress hormone)
• oestradiol
• progesterone
• testosterone
• prolactin
• thyroid hormone
• growth hormone (GH)
• insulin
• leptin (the fullness hormone)
• ghrelin (the hunger hormone)
• adiponectin
• raptin (the weight control hormone).

We’ve already talked about how light helps control the release of melatonin, which is essential for helping us fall asleep. (Just think about how turning the light on at night wakes you up.)

Many of the other hormones on that list, though, might be more surprising. Cortisol, for example, peaks shortly after you wake up [8]. Think of it like your body’s internal alarm clock. Over the course of the day, it tapers off, reaching roughly half of its peak level by the time you fall asleep [8]. Flatter cortisol rhythms – where the peaks and troughs are closer together – have been associated with various health conditions, including depression [9].

Leptin, ghrelin and raptin (a trio of hormones that influence when and how much we eat) are also controlled by your circadian rhythm. It’s one reason why poor sleep can lead to weight gain.

What Is a Circadian Rhythm Disorder?

What happens when your genetic clock pulls in one direction – but your zeitgebers pull in another? You develop a circadian rhythm disorder.

Circadian rhythm disorders include [1]:

• delayed sleep–wake phase disorder or DSPD (where you wake and sleep later than you should)
• advanced sleep–wake phase disorder or ASPD (where you wake and sleep earlier than you should)
• irregular sleep–wake phase disorder (where your sleep and wake times are consistently chaotic)
• non-24-hour sleep–wake rhythm disorder (where your baseline rhythm – your tau – drifts, becoming increasingly delayed).

Circadian rhythm disorders can lead to daytime sleepiness. They can also cause chronic sleep deprivation, which has many negative impacts, and have been linked to disorders ranging from obesity to metabolic syndrome [10].

If you think you might be living with a circadian rhythm disorder, talk to a sleep doctor. A long-standing disorder can be extremely difficult to correct without help. Circadian disorders can also be symptoms of neurodegenerative conditions like Parkinson’s and Alzheimer’s – another reason why getting medical advice matters [1].

ADHD and Circadian Rhythms

Many people with attention deficit hyperactivity disorder (ADHD) are also Wolf chronotypes with circadian rhythm disorders. Some studies have found that as many as 80% of people with ADHD are Wolves, with more than 60% reporting daytime sleepiness [12].

Why? It isn’t clear. One theory: people with ADHD make poorer lifestyle choices, which negatively affects their circadian rhythms.

Interestingly, though, almost all clinically relevant ADHD symptoms are also symptoms of poor sleep. Difficulty focusing, reacting without thinking, difficulty following instructions, risk-taking/dangerous behaviours, restlessness, stimulation seeking, and poor self-control have all been proven to be caused by sleep deprivation [13, 14, 15]. If you tested someone with chronic sleep deprivation against the DSM-5 criteria for ADHD, they would almost certainly meet the threshold for ADHD diagnosis.

It’s possible that Wolves (who are most likely to suffer from social jet lag) often get less sleep than they need. That chronic sleep loss causes ADHD-like symptoms, which lead to worse lifestyle choices that, in turn, reduce sleep length and quality [15].

Circadian Rhythm Disruptors

Like most Australians, you probably have a disrupted circadian rhythm. It’s not your fault. Living in a developed nation means we’re constantly exposed to circadian rhythm disruptors, or what sleep expert Olivia Arrezolo calls ‘sleep saboteurs’.

Those include:

• artificial light (especially screens)
• continual food availability
• stimulants like caffeine, sugar, and recreational drugs
• prescription drugs
• alcohol
• tobacco
• shift work
• chronic stress
• jet lag
• social jet lag.

We’ll discuss how you can reset your circadian rhythm by avoiding/limiting sleep saboteurs shortly.

How to Calculate Your Circadian Rhythm

The easiest way to work out your circadian rhythm is to take a free online quiz like this one.

You can also check your inner ear temperature. It’s the best way to non-invasively measure core body temperature, which is regulated by your circadian rhythm. Your temperature should be at its lowest around 3 to 5 am and its highest around 5 to 7 pm [16]. Your ideal sleep time is around 4 to 6 hours before your lowest point [17].

Try checking your temperature every hour you’re awake for a week. Make sure you maintain the same schedule – eating, air temperature, and movement can all affect your body temperature [18]. While you probably won’t be able to measure your temperature at its lowest point (that normally occurs when you’re asleep), the lowest and highest points are normally around 14 hours apart.

If you found your highest temperature occurred at 5 pm, for example, your lowest temperature would probably occur around 3 am, which means your ideal bedtime would be 9 pm.

How to Reset Your Circadian Rhythm

To reset your circadian rhythm, you need to ‘entrain’ your body using zeitgebers. As we talked about earlier, those are:

• light
• food
• temperature
• movement
• social interactions.

It’s also a good idea to avoid or limit sleep saboteurs. Those include:

• caffeine
• alcohol
• tobacco
• sugar
• recreational drugs
• naps longer than 30 minutes

Try following these tips to reset your circadian rhythm.

1. Go to bed and wake up within +/- 30 minutes of the same time each day.
2. Go outside and get direct sunlight (no sunglasses) within 30 minutes of waking up. If you can’t, use a SAD lamp instead.
3. Avoid bright lights (especially screens) at least 1.5 hours before bed.
4. Make sure your bedroom is as dark as possible. Even small electronic lights can affect your sleep.
5. Keep your room cool and use a mattress with cooling technology.
6. Stop socialising at least an hour before bed. Try to have any stressful or difficult conversations earlier in the day.
7. Avoid heavy meals, alcohol, and exercise at least 2 hours before bed.
8. Avoid caffeine and naps at least 6 hours before bed.
9. Avoid smoking and recreational drugs as much as possible.
10. Ask your doctor about the sleep impacts of any medication.

How Long Does It Take to Fix Your Circadian Rhythm?

Once you’ve fixed your daily routine, expect to wait up to 30 days for your circadian rhythm to fully adapt. Most people will experience improvements within a few weeks, but your body can take time to adjust, especially if you’ve been living with a circadian rhythm disorder for years.

Best Books on Circadian Rhythmicity

If you want more information after reading this article, think about borrowing or buying a book that covers circadian rhythms in more detail. Our top 3 recommendations are Bear, Lion or Wolf by Olivia Arrezolo, Why We Sleep by Matthew Walker, and The Power of When by Michael Breus.

Bear, Lion or Wolf

By Olivia Arrezolo

Written by Australia’s leading sleep expert, Bear, Lion or Wolf combines an accessible style with detailed recommendations for each of the 3 chronotypes.

Why We Sleep

By Matthew Walker

Dr Matthew Walker is one of the world’s most prominent sleep scientists whose 2017 bestseller reshaped the global conversation around sleep.

The Power of When

By Michael Breus

From the creator of animal-based chronotypes, Dr Michael Breus, this book includes exact timings for over 50 different lifestyle activities.

The information contained in this article is general information only. It should not be construed or used as medical or healthcare advice. For personalised advice, book a consultation with a qualified and registered medical or allied healthcare professional.