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Table of Contents


Volume 6 Issue 1 2022


Volume 5, Issue 1, 2022

Is Your Body Clock on Time?

Stephanie Osfield: Counsellor, Teacher, Health and Medical Journalist

Do you stay up late on your computer or phone? If so, your body clock could be getting out of step with day and night, which could be compromising everything from your sleep to your energy and concentration.

                      Figure 1: Irregular sleep patterns can disrupt your body clock, or circadian rhythm

Have you ever sat up almost until dawn texting a friend or studying for a big exam, then found you couldn’t sleep for nights afterwards? Blame it on disruption to your body clock – a biological pulse found in animals, plants, fungi and some bacteria

Also called the circadian rhythm, it’s the reason flowers close at night and open again in the morning and nocturnal animals like possums, sleep all day. 

In humans, our body clocks are diurnal – which means that we rest when it is dark and become active when the sun is up. Meanwhile, though we are unaware of it, our body clock is silently ticking away, influencing our appetite, energy levels, moods and behaviour.

Figure 2: All animals, plants, fungi and some bacteria have a natural biological pulse that is in tune with night and day.

Clocked On

The circadian sequence affects every biological process in the human body. Our ‘master clock’ is located in the brain, where it is run by the suprachiasmatic nucleus (SCN), an area only the size of a grain of rice. 

The SCN is located in the hypothalamus, which is like a hormone factory made up of about 20,000 nerve cells (called neurons) that sit in a cluster just above the brain stem. The hypothalamus is also linked to our optic (visual) nerve, which is why light has such an influence on our body clock. 

Figure 3: The suprachiasmatic nucleus regulates sleep and wake patterns and produces hormones such as melatonin, known as the ‘sleep hormone’.

In recent decades, scientists also made the surprising discovery that in addition to the master clock, we have individual body clocks throughout our body. These keep time in our cells and organs such as the heart, lungs, liver, kidneys, spleen, digestive system and adrenal glands.

When everything is in synch, our master clock acts like a conductor and the mini clocks follow its lead. Yet when our circadian rhythm gets out of step with night and day, the smaller body clocks can do the same and our health may be compromised.

Circadian Desynchrony

Shift work, travelling overseas to another time zone and staying up too many nights to binge-watch a series can all cause a condition called ‘circadian desynchrony’, where your body feels and behaves like it is day time when it is night. 

Desynchrony is a sign that the circadian rhythm is out of step with light and dark. If it becomes chronic, it can trigger or contribute to conditions such as depression, dementia (cognitive deficits that impede thinking), changes in appetite, dips in energy levels and trouble falling asleep (insomnia).

Circadian Cues

Our body clocks are influenced by a variety of cues and stimuli, known as ‘zeitgebers’ (which translates as ‘time givers’ in German). Zeitgebers have an enormous impact on the circadian rhythm, by switching on and off a complicated network of proteins and genes. 

These prompts pass information on to the master body clock and also influence when the small body clocks engage in certain processes. For example, sitting in darkness tells our body clocks that it is night time so that the body knows to perform certain functions such as cellular repair – which mostly happens when we are sleeping. 

The zeitgebers that have the most impact on our circadian rhythm include the following: 

1. Hormones and Body Temperature

The production of hormones is pivotal to the circadian rhythm. The hormone cortisol, which rises in the morning to help provide energy for the day, can start to be produced by our bodies as young as two months of age. 

The production of melatonin, also known as the ‘sleep hormone’, drops our temperature in readiness for sleep. In a well synchronized body clock, body temperature will drop at night when more melatonin is produced, which starts to rise soon after night falls and peaks somewhere between 2am and 4am. 

Morning light exposure sets the internal body clock of humans, signaling body temperature, energy levels and alertness to stay high because it is day time. Darkness does the opposite, telling many body systems to slow down and get ready for sleep, directing the brain to send signals to ensure the body temperature starts to drop in the evening. 

2. Light and Sleep

Before electronic light became more widely available in the late 1800s, most households relied on candle-light at night. As candles were expensive so usually in limited supply, most people went to bed soon after dark and rose for the day soon after sunrise. 

        Figure 4: Before electronic lights became available, most households relied on candle light at night                                             so people tended to sleep and wake in tune with night and day.

Now we live in a world where light from overhead globes and table lamps is available at the flick of a switch. Light also emanates from many electronic devices, including televisions, computer screens, mobile phones, alarm clocks, microwaves, fire alarms, house alarms and even digital watches. 

Light spillage trespasses into our homes from street lights, our neighbours’ outdoor or sensor lights and car lights on busy roads. Even if you do not live directly in the city, your bedroom may still have light spillage at night from the bright glow of lights left on in office buildings and emanating from huge advertising billboards. 

There is so much artificial light at night that some experts now call it light pollution. Studies show that this unnatural evening light is interfering with the circadian rhythm in many animals, causing disruption to their natural feeding, living, migratory and breeding patterns. Humans are also experiencing these disruptions due to artificial light from many sources, especially technology.

Sitting in front of screens such as laptops at close range in the evening can interfere with hormonal messages. This can then lead to delayed sleep onset, which reduces the number of sleeping hours. 

Avoiding particular colours of light at night has also been identified as important. Blue light may be the most harmful for sleep and for the circadian rhythm, according to Harvard University researchers. Their studies have found that blue light suppresses melatonin for twice as long and shifts the body clock by as much as three hours. 

Figure 5: Sitting in front of screens at night can interfere with sleep hormones and sleep cycles.

For people who have severe desynchrony, some sleep experts now prescribe a successful treatment called ‘phototherapy’. Special light boxes are used to deliver light exposure at different times of day and slowly help return a person’s body clock back to normal. The timing and intensity of the light exposure depend on whether a person falls asleep too early, experiences insomnia and can’t nod off or wakes before the night is through, around three or four am. 

Re-setting the body clock using light then helps to reset a person’s biological thermostat, which also affects and is affected by the circadian rhythm. Other small daily habits may help this re-synchonising process. They include breakfasting outside to get good early morning light exposure through the eyes and using lamp lights with low wattage globes rather than overhead lights at night. 

It is also helpful to set a technology curfew to avoid looking at bright devices like phones and laptops too close to bed time, as the light can scramble our body clocks, mistakenly telling them that it is day time when in reality it is night and almost time for sleep.

3. Food and Meal Times

The circadian rhythm of the digestive system is affected by light and dark, so it is most efficient during the daytime and least active at night. That’s good reason to eat our biggest meal at breakfast and a lighter meal in the evening. 

The liver also works to a body clock, and will start to make certain enzymes ready at different times of the day in anticipation of our next meal. For this reason, to help keep our body clock on time, we should aim to eat breakfast, lunch and dinner at similar times each day.

Figure 6: Our digestive system is affected by light and dark, and works most efficiently during the day – a good reason to avoid eating late at night.

It is important to avoid skipping meals, especially breakfast. On the flip side, eating late meals and snacks may increase the likelihood of storing fat from the food we eat because the body clock is putting the metabolism on ‘go-slow’ mode at night, which means we burn less kilojoules after dark.

Although the darkness of night may be telling you that it is time to sleep, eating food confuses this message and tells your pancreas that it is day time then directs it to start producing insulin to help keep your blood sugars stable. This can then put your pancreas body clock out of time with your SCN master body clock. 

4. Exercise

The circadian rhythm raises body temperature throughout the day, so it is usually a few degrees higher in the afternoon than the morning. At this time, muscles and joints also become more flexible, so risk of injury is lower, nerve impulses speed up, co-ordination improves and reaction times are faster. In short, you can work out harder with less perceived effort. 

Even lung function is more efficient at around 5pm than it is at midday, according to research from Albany Medical College. Muscle strength also peaks between two and six pm, improving our ability to squat, run or strike a yoga pose. It’s little surprise then that most Olympic records are broken later in the afternoon rather than in the morning.

Research from the University of South Carolina at Columbia monitored 25 experienced swimmers while they engaged in six timed trials at different times of day. Uniformly, their performance peaked in the evening and they swam slower times at 5 am. 

That does not mean that morning exercise is not beneficial – the best time of day to work out is when it is most enjoyable and can be scheduled into the day.

Yet if training for an event such as a marathon or swimming finals, it is a good idea to include some workout sessions in the afternoon or early evening. 

Exercising past six or seven pm is not recommended, as this can put the body clock out of time, causing alertness and increasing body temperature just when the opposite should be occurring to help the body and brain gear down for nightly rest.

Figure 7: Night and day have powerful influences on many bodily functions other than sleep, including energy, mood, concentration, digestion and exercise effectiveness.

Energy and Concentration

Body temperature and alertness drop overnight, particularly between two am and four am, and alertness is lower at these times. Subsequently, this overnight period has been shown to be a time when fatal car crashes and human errors that cause accidents are more likely to occur. 

When body temperature and alertness are low from body clock signals, people do not think as clearly and as a result, may make poor decisions that have serious consequences. Studies show that even just changing our clocks by one hour during daylight savings can lead to more traffic accidents.

In the morning when body temperature starts to rise, alertness should also increase. Research suggests that concentration has a particular peak around 10 am, so if you are studying on the weekend, this is an optimal time to write reports and complete essays and assignments. Verbal skills may peak around midday, so people in business may benefit from scheduling meetings then. 

Most people have a slight temperature dip between one and two pm, which can bring on tiredness. To accommodate this, some forward-thinking companies now provide nap rooms for workers to use, in the knowledge that a short afternoon siesta can help improve focus and energy for the rest of the working day.

Figure 8: The suprachiasmatic nucleus regulates sleep and wake patterns and produces hormones such as melatonin, known as the ‘sleep hormone’.

Body Clock and Body Shape

Staying awake far past nature’s bed time goes against the natural circadian rhythm. If we do this repeatedly it could cause weight gain. Sleeping less because of staying up well beyond midnight can put the body clock completely out of step with night and day. When this happens, we may burn kilojoules less efficiently. 

Therefore, in the future, watching light exposure may be considered just as important to a healthy weight as a balanced diet and regular exercise. Circadian desynchrony may be scrambling the systems in our brain that regulate metabolism and hormones such as insulin, which keep the blood sugar stable, shows research from the Chronobiology Research Group at the University of Aberdeen. 

In the long-term, the resulting circadian desynchrony may increase the risk of developing conditions like diabetes and obesity. In a report called, “Living against the clock; Does loss of daily rhythms cause obesity?”, researchers in this area suggested that circadian desynchrony needs more investigation as a potential cause of obesity.

Student activities

1. What is a circadian rhythm?

2. Discuss how their body clock affects three different living organisms.

3. List four cues (zeitgebers) that can impact your body clock.

4. Identify five sources of artificial light that could be affecting your circadian rhythm in the evening. Discuss what impact this light can have on sleep.

5. What is circadian desynchrony?

6. Draw two pictures on one page – one showing night and one showing day time. Write down some bullet points on the relevant side to show how your body clock is affected by the hormones melatonin and cortisol.

7. Keep a thermometer by your bed every morning and take and write down your temperature first thing when you wake, before you get out of bed. Then take your temperature last thing before you go to bed. Note which temperature is lower and what this indicates about your body clock and whether it is in step with light and dark.

8. Set a bright light curfew every night for three nights and only use lamplight or electric candles from 7pm on. Keep a diary recording what time you feel tired enough to go to bed. Then remove the light curfew and use overhead lights all evening. Record whether you feel tired later in the evening. Compare the results.

9. Make a table with two columns listing the differences between the behaviour of animals that are nocturnal and diurnal.
10. Take a photograph of a flower in the morning and then later in the afternoon to show how its body clock affects its opening and closing.

References and Resources

Circadian Rhythms

ABC News. Your Body Clock, Or Circadian Rhythm, Can Affect Your Health If It Gets Out Of Synch. Here’s How To Manage The Risk:

CellPress Current Biology Review, The Circadian Clock and Human Health:

Cleveland Clinic. Do You Need to Reset Your Circadian Rhythm? 2021:

Crash Course Video: Circadian Rhythm and Your Brain’s Clock.

Harvard Health. Blue Light Has a Dark Side: 

Harvard Medical School, Circadian Rhythms and the Brain:

Mayo Clinic. Bright Light Therapy:

National Institute of General Medical Sciences Physiology. Circadian Rhythm:

NSW Government, Light Pollution: Foundation, Circadian Rhythm: Post. What Is The Best Time Of Day To Exercise? It’s Not What You Think:

Akihiro Goriki, Fumiyuki Hatanaka, Jihwan Myung, Jae Kyoung Kim, Takashi Yoritaka, Shintaro Tanoue, Takaya Abe, Hiroshi Kiyonari, Katsumi Fujimoto, Yukio Kato, Takashi Todo, Akio Matsubara, Daniel Forger, Toru Takumi. 2014. A Novel Protein, CHRONO, Functions as a Core Component of the Mammalian Circadian ClockPLoS Biology, 2014; 12 (4): e1001839 DOI: 10.1371/journal.pbio.1001839

Buhr, E. D., Yoo S. H., Takahashi, J. S. 2010. Temperature as a Universal Resetting Cue for Mammalian Circadian OscillatorsScience, 2010; 330 (6002): 379 DOI: 10.1126/science.1195262

Cogswell, D., Bisesi, P., Markwald, R. R., Cruickshank-Quinn, C., Quinn, K., McHill, A., Melanson, E. L., Reisdorph, N., Wright, K. P., Depner, C. M. 2021. Identification of a Preliminary Plasma Metabolome-based Biomarker for Circadian Phase in HumansJournal of Biological Rhythms, 2021; 36 (4): 369 DOI: 10.1177/07487304211025402

Crosby, P., Hamnett, R., Putker, M., Hoyle, N. P., Reed, M., Karam, C. J., Maywood, E. S., Stangherlin, A., Chesham, J. E., Hayter E. A., Rosenbrier-Ribeiro, L., Newham, P., Clevers, H., Bechtold, D. A., O’Neill, J. S. 2019. Insulin/IGF-1 Drives PERIOD Synthesis to Entrain Circadian Rhythms with Feeding TimeCell, 2019; DOI: 10.1016/j.cell.2019.02.017

Heyde, I., Oster, H. 2019. Differentiating External Zeitgeber Impact on Peripheral Circadian Clock Resetting. Scientific Reports, Article number: 20114 (2019). 

James, S. M., Honn, K. A., Gaddameedhi, S.,Van Dongen, H. P. A., H. 2017. Shift Work: Disrupted Circadian Rhythms and Sleep—Implications for Health and Well-Being, Current Sleep Medicine Reports Journal, 2017 Jun;3(2):104-112. doi: 10.1007/s40675-017-0071-6.Epub 2017 Apr 27.

Khullar, A., The Role of Melatonin in the Circadian Rhythm Sleep-Wake Cycle. 2012. Psychiatric Times, Vol 29 No 7.

Kline, C. E., Durstine, J. L., Davis, J. M., Moore, T. A. 2007. Circadian Variation in Swim Performance. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology, 102(2):641-9 DOI:10.1152/japplphysiol.00910.2006

Rivkees, S. A., The Development of Circadian Rhythms: From Animals To Humans. Sleep Med Clin. 2007 Sep 01;2(3):331-341

Roenneberg, T., Allebrandt, K. V, Merrow, M., Vetter, C. Social Jetlag and Obesity. Current Biology, 2012; DOI: 10.1016/j.cub.2012.03.038

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