Circadian Rhythm: The Science Behind Your Internal Clock

Circadian rhythms are the daily cycles of our bodily processes, such as sleep, appetite, and alertness.

In a sense, we all know about them because we all experience them. We feel more alert at some points in the day and tired at others.

But we may not understand just how our internal clock works, how important it is, or how we can keep it working at its best. Read on to find out all about your circadian rhythm and how to work optimally according to your circadian clock.

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Understanding How Circadian Rhythms Work

It has always been obvious that humans and other organisms are attuned to the 24-hour cycle of the Earth’s rotation. What is less obvious is that these daily cycles are not only a response to environmental cues, such as the rising and falling of the sun, but are also driven by an internal clock (Albrecht, 2012).

Circadian rhythm: A definition

Our internal clock is called “circadian” because it means “roughly a day,” and the clock, without external cues such as the rising and setting of the sun, defaults to a cycle of 24.2 hours per day (Czeisler et al., 1999).

With external cues, it is reset daily to an exact 24-hour cycle to keep time with the Earth’s 24-hour rotation.

Entrainment

The process by which the biological clock is reset to exactly 24 hours is known as entrainment, and it happens via zeitgebers (which translates as “time-giver”). These are the environmental cues that our bodies use to tell what time it is, and the most important of them is daylight (Albrecht, 2012).

Circadian clock vs. biological clock

The terms circadian clock and biological clock can be used interchangeably. An organism’s circadian or biological clock is the mechanism within the organism that controls its circadian rhythms. It is a complex system of proteins in cells throughout the body, controlled by thousands of genes (Klein et al., 1991).

In vertebrates, a master clock regulates these genes and proteins. In humans, this is a structure in the brain called the suprachiasmatic nucleus (SCN; Klein et al., 1991).

The SCN communicates with “clocks” in other parts of the body (e.g., the heart and liver) to make sure that all the tissues of the body are in sync with it (Dibner & Schibler, 2015).

Why Circadian Rhythm Is Crucial for Mental and Physical Wellbeing

Circadian rhythm is something that we tend to take for granted, until it goes wrong. Not only does circadian disruption feel bad, but worse, over the long term, it is detrimental to health.

Here is researcher Satchin Panda with a helpful overview:

Sleep

Our circadian rhythms are crucial to good sleep, and good sleep is crucial to good health. Long-term sleep disruption is associated with obesity, diabetes, cancer, and cognitive decline, among other issues (Knutson & Van Cauter, 2008; Mogavero et al., 2021; Wennberg et al., 2017).

Under optimal circumstances, the hormone cortisol peaks shortly after you wake up, setting the timer for the cycle of alertness and tiredness that will lead you back into sleep many hours later (Dickmeis, 2009).

But that process can easily be disrupted. Sunlight is the primary cue for the body to produce that spike in cortisol, although eating and physical activity also contribute. Without exposure to sunlight within the first hour after waking, our circadian clock might not be properly entrained. Artificial lights, which we are more likely to see in the morning, are not usually bright enough to do the job (Potter et al., 2016).

And it gets worse. While we see too little light in the morning, we see too much in the evening. The artificial lights of our homes, mobile phones, and computer screens blaze brightly in the evening after the sun has gone down, cueing our circadian clock to keep us unnaturally alert so that we do not sleep as early or as deeply as we should (Potter et al., 2016).

Optimal bodily functioning

As well as our sleep, circadian rhythms influence various aspects of metabolism such as hormone release, appetite, digestion, and body temperature, all of which are essential to the proper functioning of our bodies (Potter et al., 2016).

Disruptions of these systems due to disrupted circadian rhythms increase the risk of physical health problems, including cardiovascular disease, obesity, diabetes, and cancer. This happens partly due to the direct effects of circadian disruption on metabolism and partly due to its influence on food preferences. People are more tempted by fatty and sugary foods when sleep deprived (Simon et al., 2015).

Performance

As might be expected, the effects of circadian rhythm on the functioning of our bodies have implications for both physical and mental performance.

In one study, researchers found that swimmers were fastest 90 minutes after the drop in body temperature that happens each evening (Kline et al., 2019). In others, they have found that athletes showed diurnal (daily) variation in key indicators of performance such as VO2 max, strength, flexibility, sprint speed, and reaction time (Knaier et al., 2019; Vitale & Weydahl, 2017).

As regards mental performance, there is evidence that cognitive functions are affected by arousal levels, which are in turn affected by circadian rhythms. This is particularly true for those cognitive functions that are involved in top-down cognitive control, such as inhibition, working memory, task switching, and psychomotor vigilance (Xu et al., 2021).

Mental health

Disturbed circadian cycles are the rule rather than the exception across all categories of mental illness (Baglioni et al., 2016). In one study of 121 adults receiving treatment in a community mental health center, over 85% met criteria for two or more sleep-circadian problems (Sarfan et al., 2021).

The relationship between circadian rhythm and mental health seems to run in both directions. Disruptions to circadian rhythm contribute to mental health problems, while mental health problems in turn contribute to disrupted circadian rhythms (Meyer et al., 2024).

There are several mechanisms that might contribute to this relationship, including lack of exposure to daylight, disturbance to sleep and its important role in emotional processing and neuroplasticity, and effects on the microbiome (Meyer et al., 2024).

Insomnia and hypersomnia (which imply circadian disruption) are so common in depression that they are part of the diagnostic criteria for the condition (World Health Organization, 2019), and around a third of people with major depression experience both (Geoffroy et al., 2018).

Alongside these gloomy facts, however, we can set the prospects for treating depression with interventions targeting the circadian clock. There is evidence that exposure to very bright artificial light in the morning can be effective in treating both seasonal depression (or seasonal affective disorder) and depression at large (Lewy et al., 2006; Al-Karawi & Jubair, 2016).

As with other mental health problems, there is a strong link between anxiety and sleep disorders, with 24% of those with anxiety disorders also experiencing insomnia, and 28% experiencing hypersomnia (Ford & Kamerow, 1989).

Again, as with other mental health problems, causality is likely to run in both directions. It is hard to fall and stay asleep when anxious. Whereas disturbed sleep means impaired emotional processing during sleep, which means, in the daytime, more anxiety, hyperarousal, rumination, and emotional reactivity (Van Someren, 2021).

Circadian rhythm disorder

While circadian rhythm disorders are classified in the International Statistical Classification of Diseases and Related Health Problems (World Health Organization, 2019) as sleep–wake disorders rather than mental and behavioral disorders, they are, as set out above, very often associated with mental health problems of various kinds.

A circadian rhythm disorder is a problem that occurs when your circadian rhythm is out of step with your environment. They include (National Heart, Lung, and Blood Institute, 2022):

• Advanced sleep–wake phase disorder: Going to bed very early and rising very early
• Delayed sleep–wake phase disorder: Going to bed very late and rising very late
• Irregular sleep–wake phase disorder: When sleep is broken up into several short periods throughout the night and day
• Non-24-hour sleep–wake phase disorder: When the sleep–wake cycle is not 24 hours long
• Jet lag disorder: When crossing time zones while traveling leaves you out of sync when you arrive at your destination
• Shift work disorder: When switching between day and night shifts leaves you out of sync with your working hours or the world around you

Acknowledging Individual Differences in Circadian Rhythms

It is readily observable that people have different circadian rhythms — most obviously, morning people vs. night owls. These different profiles are referred to as a person’s chronotype (Roenneberg et al., 2003).

A person’s chronotype is not fixed, but can change under the influence of factors such as light exposure, season, routine, and age. Young children typically get up early, whereas teenagers (famously) can’t be roused before midday. There is, however, a genetic influence on chronotype, which lends the trait a degree of stability and means that it runs in families (Kalmbach et al., 2017).

How to identify your personal rhythm

Many of us can say quite easily whether we are a morning person or an evening person, but for more rigorous assessment, there are a number of scientifically validated questionnaires developed by researchers.

These include the Munich Chronotype Questionnaire (Roenneberg et al., 2007), the Morningness-Eveningness Questionnaire (Horne & Östberg, 1976), and the Composite Scale of Morningness (Smith et al., 1989).

The Effects of Light Exposure on Circadian Rhythm

Our bodies use a range of cues to set their internal clocks.

The most important of them is light, which is received by specialized cells in the retina called retinal ganglion cells, which feed information on light levels into our central circadian clock, the SCN in the brain (Blume et al., 2019).

The precise effect of light upon the circadian clock, however, depends upon both the time of day of light exposure and the frequency (i.e., color) of the light (Wahl et al., 2019).

Morning light

As mentioned above, exposure to bright light in the morning advances the circadian clock; it brings sleep closer. Or, to put it another way, it tells the body that daytime has arrived and sets the internal clock ticking toward nighttime and bedtime. Not any light will do, though. Daylight is massively brighter than most artificial lights, and it is daylight that our circadian system has evolved to respond to (Spitschan et al., 2016).

Evening light

Exposure to light in the evening, meanwhile, tricks the internal clock into behaving as though it is still daytime, which delays the onset of sleep. Unfortunately, this response can be triggered by artificial light, so switching on the lights or looking at a computer or phone screen in the evening is likely to keep you awake.

Blue light

Blue light has a short wavelength and has the largest effect on circadian rhythm. This is because the pigment melanopsin, which is found in the retinal cells that affect circadian rhythm, is most responsive to light at this frequency (Wahl et al., 2019).

Red light

Red light has a longer wavelength than blue light and so has less effect on the melanopsin pigment in the eye — and therefore on circadian rhythm (Tähkämö et al., 2019).

2 Scientific Protocols for Resetting Your Circadian Clock

If you are alarmed by the news that disruption to circadian rhythms is bad for your health and yet common under the conditions of modern life, then be assured that there are ways to combat it.

Jetlag

The most effective way to combat jet lag is to cause a phase shift, which is to say, to move your circadian rhythm earlier or later to bring it into line with your destination. An effective way to cause a phase shift is to expose yourself to bright light either in the morning, to make you sleep and wake up earlier, or late in the day to make you sleep and wake up later. This can be done either before traveling or after.

The procedure goes like this (Burgess et al., 2003):

1. Determine when your body temperature reaches a minimum. This is usually two to three hours before you wake up.
2. If you need to bring your circadian cycle forward to wake earlier, expose yourself to daylight or an artificial light box for four hours before your body temperature minimum and avoid it for four hours afterward. If you need to delay your circadian cycle, do the opposite.
3. Aim to move the timing of your minimum body temperature by one hour each day if you are trying to bring your circadian cycle earlier and by 1.5 hours if you are trying to make it later.

As a handy tip, there are several jet lag calculators online and even a jet lag app for frequent travelers.

Shift work

Shift work can be especially detrimental to a healthy circadian rhythm because it is often a long-term arrangement. Helpfully, the Centers for Disease Control and Prevention offer evidence-based training for nurses working night shifts, in which are included these suggestions (Jones & McAuliffe, 2023):

1. Prioritize sleep. Go to sleep straight after a night shift, rather than giving in to the temptation to take care of chores and other business.
2. Spend as much time as possible in brightly lit areas during the first half of a night shift and more time in darker areas during the second half, to encourage the transition to a new circadian rhythm.
3. Try blue light-blocking glasses when traveling home from a night shift in daylight. This won’t completely eliminate the effects of daylight on your circadian clock, but it helps.
4. Make your bedroom as dark as possible for sleep.
5. As a last resort, consider taking a melatonin supplement close to bedtime. This is a hormone that helps induce sleep.

Enhance Your Circadian Cycle With These 6 Tips

Light exposure is the single biggest zeitgeber, but here are a few more habits to strengthen your circadian rhythm, courtesy of Andrew Huberman (2021), whose podcast on optimal health and performance has done a lot to bring circadian rhythm and good sleep hygiene to wider public attention.

1. Exercise early in the day. Intense physical activity later on will push your bedtime back.
2. Consume caffeine early in the day, and not too much.
3. Don’t eat too much in the evening or at night. Food consumption is a signal to your body that it’s time to be active and alert.
4. Don’t drink (much) alcohol. Alcohol might help you fall asleep, but it spoils the quality of your sleep, which will throw off your circadian rhythm.
5. Don’t nap for too long or late in the day.
6. Consider medication and supplements. We’ve already mentioned melatonin, but Huberman (2021) instead suggests supplements such as magnesium threonate, apigenin, and theanine.

A Take-Home Message

Hopefully, by now you have some idea of both the importance of circadian rhythms and what you can do to preserve or improve yours.

It’s a challenging task because so much of modern life, with its brightly lit evenings, alluring devices, and stimulating caffeine, seems designed to pull us out of our natural rhythm.

But armed with the knowledge and tools in this article, you can seek a balance between doing the things that you want to and taking care of yourself.

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