Circadian rhythm and sleep: Everything You Need to Know

Circadian Rhythm and Sleep: Everything You Need to Know

The complete guide to circadian rhythm effects on sleep, hormones and health

In this post, you’ll learn about the circadian rhythm and how it impacts sleep.

Let’s dive in right away.

What is the circadian rhythm

Our body has an internal clock that enables us to fall asleep and wake. It’s called the circadian rhythm. 

The circadian rhythm plays a crucial role if you want to fall asleep on time

The chief influencers of the circadian rhythm are light and darkness in your environment.

The circadian rhythm is, in essence, our body’s timekeeper for sleep. 

For example, the circadian rhythm helps our bodies to respond to environmental cues that inform our bodies to feel sleepy or awake.


Those sleep-wake responses are influenced by light or the lack thereof.

But it’s not just sleeping that gets affected. 

Circadian rhythm overall tracks the changes in our psychological.

Whether it’s mental wellbeing and physical state, this happens when the internal clock aligns with a daily cycle

It’s not only humans that have circadian rhythms. 

Animals, plants, and other living organisms have it too. The term for studying the circadian rhythm is chronobiology.

What is the biological clock?

A biological clock refers to an intrinsic sense of time in all living organisms. 

That sense of time is a result of complex cellular interactions in the body. The main actors in this interaction are protein molecules.

Most living organisms have a biological clock. 

The same genes responsible for the biological clock signals in humans are also present in mice, fruit flies, and fungi.

Our biological needs resetting to align with the rotation of the earth. This is mainly because the clock is not 24 hours.

This also means that each person’s timing could be different depending on location and lifestyle habits.

Can you reset your circadian rhythm?

In general, the circadian rhythm follows a cycle of around, but not exactly 24 hours. 

It’s not 24 hours up the microsecond – that’ll be too serendipitous. 

To help us align with our environment, light, and a sense of time each day, the circadian system needs to be reset.

In most cases, the process of resetting or synchronizing happens naturally as we expose our light during the day and darkness at night.

Circadian rhythm chart example

Researchers from Harvard Medical School and Brigham & Women’s Hospital did a study. 

They looked into the human’s response to the circadian system. 

Using experiments, they shifted the timing of the sleep-wake cycle. This enabled them to present light stimuli across the entire 24-hour circadian cycle.

Source: Sleep Medicine Clinics Journal

The findings showed the critical role of light stimulus in biological day and night.

How does the circadian rhythm impact sleep?


Exposure to light or darkness regulates the circadian system. Here’s why.

Our biological clock controls a part of the brain called suprachiasmatic nucleus (SCN). 

The SCN influences our reactions to light and darkness. It does this by controlling the production of melatonin. Melatonin is what makes us sleepy.

The SCN gets information about light from the optic nerves in the eyes. It then passes this signal on to the brain.

If there’s darkness (no light) around, SCN informs the brain to produce melatonin so you’re sleepy. 

When there is light, SCN informs your brain to suppress melatonin so you’re alert and awake.

Related: 5 Benefits of Sleep That Never Go Out of Fashion

Does light exposure affect the circadian system?

When you expose yourself to light at night, it can affect your body’s response. You become less sleepy. And more awake. 

This is especially true with blue light. 

In general, blue light is helpful during the day. This is because it stimulates our mood, attention, and energy

But it can also be detrimental at night when we want to relax before bed and have a restful sleep.

One study researchers compared blue light and green light. Especially how they impact the circadian rhythm after six and a half hours of exposure.

They found something remarkable. 

Blue light is twice as likely to repress melatonin and disrupt our circadian rhythm compared to green light.

Even using our smartphones before bedtime affects our circadian rhythm. 

Not only does the light suppress sleep, but it also makes it harder to fall asleep. 

It also makes you less alert in the morning when you’re supposed to be refreshed.

The key is to use dimmer lights or red lights at night if you need to check your phone. 


This goes a long way to improve your health.

How does the circadian rhythm impact jetlag?

Jet lag is a major sleep disorder. The circadian rhythm is the body’s clock that influences sleepiness and wakefulness. 

This clock controls a part of the brain called suprachiasmatic nucleus (SCN). The SCN influences our reactions to light and darkness.

Besides controlling the sleep and wake cycles, the circadian also affects the hormones and temperature levels in the body. 

When we travel across timezones, the SCN receives visual cues of light and darkness from the eyes. 

This affects the circadian timer. 


The circadian system is not able to keep up with the speed and shift that comes with air travel across several timezones. 

Does the circadian rhythm affect hormones?

It does. This is because circadian rhythms suppress or activate the pineal hormone, melatonin.

Studies have shown that melatonin has some anti-inflammatory, antioxidant, and anticoagulopathic properties. 

The lack of it could affect certain hormonal functions in the body.

This can affect our defense against some neurodegenerative disorders. For example Alzheimer’s disease and Parkinson’s disease.

Does the circadian rhythm impact health?

Besides hormonal changes and the impact on sleep, the circadian system has more health consequences.


The circadian rhythm can weaken the body’s immunity. It affects vital body functions like body temperature, heart rate (lower at night), and blood pressure.

When these functions are directly affected, it can result in several health conditions. 

For example, colder body temperature may lower your body’s defense and makes you more susceptible to the common cold or flu.

When your circadian rhythm is out of sync for a long time, it makes you vulnerable to sleep disorders. But that’s not all. 

It also weakens your defense against other chronic health risks. For example diabetes, obesity, chronic tiredness, and depression.

Nearly all sleep disorders rise and fall on the circadian system. 

It means your biological clock is out of sync with your environment. 

The result is often you’re tired and sleepy during the day. Or you’re unable to fall asleep at night. Insomnia and parasomnia like sleep paralysis are examples of these disorders.

Take for example jet lag. This is a sleep disorder where your body’s internal circadian rhythm is out of sync with your external environment. 

The key is to make sure you reset your circadian system to alight with your environment.