What’s a Circadian Rhythm?


Like many areas of science, the study of circadian rhythms has developed its own methods and jargon, making it quite difficult for newcomers and lay-people to break into the field. So, for those who aren’t already an expert, here’s a crash course in circadian biology:

A circadian rhythm is any biological or behavioural process that undergoes rhythms, with cycles lasting about one day. Since that definition is pretty wordy, let’s look at a concrete example; the sleep-wake cycle.

As you’re all aware, most humans fall asleep during the night, wake up in the morning and work during the day. Since we fall asleep, wake up and work around the same time each day, we can say that the sleep wake cycle displays a predictable rhythm. And since the rhythm repeats itself every 24 hours, we call it circadian (from the Latin “about one day”).

Your body’s clock

One important feature of the circadian system is that the cycles are driven from within the organism, and is not simply a response to light, sound, or other stimuli. As anyone who’s been jet-lagged can tell you, their body will tell them to sleep when they “should” be tired regardless of what the outside’s lights and sound tell them.

But how can your body “tell” when it’s day and night? There is a master clock inside your brain, called the Suprachiasmatic Nucleus (which is frequently abbreviated to SCN), which sends signals to the rest of the body’s cells, and those cells then synchronize their own “clocks” to it. The actual mechanism that generates the 24 hour cycle in these cells is quite complex, and is a story for another time.

Resetting the clock

Even though the rhythms are driven by the body’s clocks (or endogenously generated, for those who want to impress friends at a party), they do not ignore outside signals. What happens is that the body receives outside timing signals (called Zeitgebers, from the German “time-giver”) such as light and temperature, and the SCN synchronizes itself to them. The fancy, scientific word for this system of aligning the body clocks to outside cues is entrainment.

So, in the previous example of a jet-lagged person, their body’s SCN master clock will initially be out of sync with the light-dark cycle. But as time goes on, the light signals will be picked up, and the SCN will change its activity to match that of the outside. The SCN will then use its influence to make sure the rest of the body’s cell clocks will be adjusted to the new rhythm, and voilà, the poor person is finally back in sync with the world.

Conversely, if someone were to stay up at night, reading from a computer that’s shining light into their system, then their body will adjust its rhythm to be awake in the night, and to be tired during the day.


Why do we have circadian rhythms? Well, as beings who are so dependent on light, it makes sense for us to confine our activity to the times when we can see. Our body raises our blood pressure, heart rate, stress hormones, and sets our metabolism ready to burn food for energy, all to be ready to make the most of the daylight. At night, our body winds down, and gets ready to grow larger, fix itself, and prepare for the next day of activity.

…and Disease

Unfortunately, in modern society, we frequently interrupt our circadian system by staying up late, waking up early, jet-lagging our bodies, and working night shifts. This causes a disruption in the body’s carefully tuned circadian rhythms.

You may be asking, “So what if there’s a split between person’s circadian time and the rest of the world”? Well, aside from the difficulty waking, lack of productivity and sleepiness you get from a misaligned sleep schedule, circadian rhythm disruption throws your metabolism into a state similar to diabetes, and is linked to the development of several types of cancer, including breast, endometrialprostate and colorectal tumours.

Not only that, it has been shown to cause damage to the heart and kidneys in hamsters. This causal effect is reflected by the increase in risk of heart attacks and high blood pressure in shift workers, whose jobs require circadian disruption. Also, messing up the circadian rhythm after a heart attack leads to worse scarring and prevents the heart from repairing itself.

The effects of circadian disruption are not limited to conventional illness. Many psychiatric diseases are associated with circadian disturbances, including depression, bipolar disorder and schizophrenia.

Aside from the problems that occur due to circadian misalignment, it turns out that even normal, working circadian clocks can lead to disease.

In the 1980s, it was found that heart attacks and strokes tended to occur in the mornings much more often than other times of day. What’s going on? The normal things your body does to prepare you for the day (including the aforementioned increased blood pressure) create the perfect storm to make blood clots, which can get lodged into and block an artery supplying the heart or brain with oxygen and nutrients, leading to heart attacks and strokes.

What we can do about it

Clearly, the circadian system is implicated in many common and debilitating diseases. What can we do about it? Well, a new field of medicine call Chronotherapy is emerging, where we take advantage of the circadian system to boost the effectiveness of our healthcare.

It has great potential, because most of the medicines that the World Health Organization has deemed essential work by affecting circadian controlled targets, meaning that we can find the time of day where they have the strongest effect for the patient’s health.

One classic example of Chronotherapy is that ACE inhibitors (a type of drug used to lower blood pressure and help in heart failure) work much better when given during the sleep period than the wake period.


So there you go; circadian rhythms are daily biological rhythms that keep your body adjusted to the Earth’s 24 hour cycle. They’re controlled by a part of your brain called the Suprachiasmatic nucleus, which can take in information from the outside world to reset itself. Disrupting the circadian system is both unpleasant and unhealthy, and even working circadian clocks can lead to disease. To deal with that, we can take advantage of the system to maximize our the effectiveness of our medicines and keep us healthy and happy.


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