Circadian Rhythm

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Ever wondered why you feel energized and alert during the day and then fall asleep at night? Have you ever wondered how your body seamlessly adjusts to different time zones when you travel around the world?

The answer lies within the intricate workings of our biological clock or internal timekeeper, known as circadian rhythms.

In this article, we will understand its importance, functioning, and why it plays a significant role in overall health and mental well-being.

What Are Circadian Rhythms?

Circadian rhythms are like internal clocks that exist inside our bodies. They regulate our daily sleep cycle, wakefulness, and various metabolic functions. Think of it as a natural rhythm that keeps everything in sync.

The same as we have a 24-hour period with daytime and nighttime, our bodies also follow this pattern. These rhythms are controlled by a part of our brain called the suprachiasmatic nucleus (SCN), which acts as the master clock.

As a result, it keeps our internal clocks running smoothly by taking cues from the environment.

When the sun is out and it's bright outside, our bodies release hormones like cortisol, which keep us awake, alert, and active. It is during this time that we are the most energized and ready to face the day.

In the evening as the sun sets and darkness falls, our bodies start producing melatonin, a hormone that makes us feel sleepy. This signals to our bodies that it's time to wind down and rest. Our body temperature also drops, and our muscles relax, preparing us for a good night's sleep.

These cycles repeat every day, and they help regulate not just our sleep, but also other important bodily functions. These rhythms influence our digestion, metabolism, and even our mood.

These rhythms are found in mammals and most living organisms, including animals, plants, and even some bacteria. They play a vital role in many aspects of health and well-being.

How Does Circadian Rhythm Work?

Circadian rhythms are regulated by a complex network of nerve cells in the brain also known as the master clock. This collection of neurons or nerve cells is called the suprachiasmatic nucleus (SCN). The SCN is situated in the hypothalamus part of the brain and is responsible for managing the body's internal clock.

The SCN receives input from special cells in the eye when there is light or darkness. With the help of this information, the SCN synchronizes the time of day when you feel awake and when you feel sleepy.

Similar signals are used in SCN that trigger the production of a hormone called melatonin. When the light fades, the signals are sent through your eyes, and melatonin is released into your body, making you feel sleepy.

Similarly, other hormones like cortisol, insulin, leptin, vasopressin, and acetylcholine make you feel alert with exposure to light.

Metabolism also plays a role in regulating circadian clock. For example, nutrients such as glucose and fatty acids are more easily metabolized during the day and contribute to the production of hormones such as insulin and leptin than at night.

What is the Effect of Circadian Rhythm on Sleep?

The circadian rhythm plays a key role in regulating sleep patterns. Various hormones and neurotransmitters are produced by the suprachiasmatic nucleus (SCN) in the brain to regulate sleep.

Melatonin is one of the most important hormones involved in the regulation of sleep. It is produced by the pineal gland in response to the darkness at night. As the level of melatonin increases, it reduces alertness and makes you feel sleepy.

On the other hand, exposure to bright light inhibits melatonin production and makes you more wakeful and alert.

There are other hormones and neurotransmitters, like serotonin and GABA (gamma-aminobutyric acid) that also help in the regulation of sleep.

Serotonin is a neurotransmitter that regulates physiological processes like regulation of mood, appetite, and sleep. It inhibits the activity of certain nerve cells and reduces alertness.

GABA is an inhibitory neurotransmitter that helps in the regulation of sleep, anxiety, and muscle tone.

Along with melatonin, both serotonin and GABA are responsible for the regulation of the circadian clock.

What affects Circadian Rhythms?

Circadian rhythms are affected by external as well as internal factors.


It is the most important external factor that can cause circadian disruption. Exposure to light during nighttime when the body is preparing for sleep can adversely affect it.

Sleep Schedule:

Irregular sleep schedules and patterns can affect your quality of sleep and sleep-wake cycle.

Social jet lag

This usually happens when sleep gets affected due to social factors. As a result of social factors, sleep patterns are not aligned with the internal body clock which can lead to disruptions in circadian rhythms. As a result, fatigue and other health problems can occur, such as obesity and cardiovascular disease.

Shift work

Professional demands and timings like night shifts can impact circadian rhythm. The individual stays awake at night and tried to compensate for sleep during the day.

Jet Lag

Jet lag occurs when someone travels across different time zones, causing their natural sleep-wake cycle to become out of sync. As a result, it can disrupt normal hormonal production, metabolism, and other physiological processes in the body. 

Night Owls:

Night owls are individuals who prefer staying up late and waking up later in the morning. For example teens, they have more tendency to be night owls but this chronotype or preference can be genetic as well. Night owls tend to have higher levels of the hormone melatonin and are prone to experiencing social jet lag.


When people get older, their circadian rhythm can shift, resulting in changes in sleep patterns. Hence circadian rhythm of a younger individual can vary from an older person.

Does Circadian Rhythm change with Seasons?

Yes, the circadian rhythm can change as the seasons change. During winter, many people experience a condition called seasonal affective disorder (SAD). It is a form of depression caused by changes in circadian rhythm. 

In winter, the days are shorter and the nights are longer than in the summer. It can cause us to feel tired and fatigued since our bodies are used to getting more light during the day.

The change in daylight hours and the drop in temperature can affect their sleep patterns, appetites, and energy levels. The changes are not only seen in humans, but they also occur in plants and animals as well.

Sleep enough and expose yourself to natural light in the morning in order to maintain a healthy circadian rhythm.

How do Circadian Rhythms differ between people, and why?

Circadian rhythms are the natural physical and behavioral changes that occur in living organisms in response to the light-dark cycle. These rhythms help organisms to adapt to the changing environment and can influence sleep patterns, hormone production, and other important functions.

People's circadian rhythms differ for a variety of reasons. Some are genetic, while others can be attributed to environmental factors, such as light exposure or work schedules.

Genetics can have a significant effect on an individual's circadian rhythms. In some people, the length of time it takes for their body's internal clock to complete one full cycle may naturally be longer or shorter. This can affect the timing and rhythm of their sleep and other activities throughout the day.

In addition to genetics, circadian rhythms can be influenced by other factors such as age, sex, and lifestyle. 

Melatonin is a hormone that is responsible for regulating sleep patterns in the body. Older people have difficulty falling asleep and staying asleep because their bodies produce less melatonin.

During pregnancy or menopause, women experience changes in their circadian rhythms. Similarly, people who shift work or have irregular sleep schedules also have different sleep-wake cycles.


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