What is Circadian Rhythm?


Are you tired during the day and too awake to sleep at night? Does this get worse when your work schedule changes abruptly, or when you travel by plane on a far-away vacation – making it difficult to be alert for work and active for your holiday fun?

While searching for solutions to your irregular sleep habits, you may have heard of something called a “circadian rhythm,” which has a certain amount of influence over when you are sleepy and when you are alert.

what is circadian rhythm

But just what is circadian rhythm? How does the circadian cycle influence your sleep and your health? What influences the circadian rhythm, and what can you do to impact your circadian clock for a better relationship and experience with sleep?

Let’s get into all those helpful details and more below.

What is Circadian Rhythm?

“Circadian” comes from the Latin roots for “about” and “day,” and refers to something that recurs naturally on a 24-hour cycle.

A circadian rhythm is, therefore, the way your body changes physically, mentally, or behaviorally in a daily cycle, responding mainly to how light or dark the environment around you is.

Most living things, including plants, animals, and microbes, have a circadian rhythm. For humans, a circadian rhythm usually dictates that we sleep during the night and be awake during the day.

Studies have found patterns of many different biological activities that are linked to this daily cycle, such as hormone production and brain wave activity.

Circadian Rhythm vs. Biological Clock

You may have also heard the term “biological clock,” used to describe the way we sleep at night and are awake during the day.

Biological clocks, however, are not the same thing as circadian rhythms. A biological clock is the innate timing device inside of a living thing, found in molecules in cells all throughout your body.

Your biological clocks are what produce and regulate your circadian rhythms. A “master clock” in your brain coordinates all of these biological clocks together, keeping them in sync.

In the human brain, the master clock is a part of the hypothalamus, which receives input from the eyes – which is why our circadian rhythms are so tied to the presence or absence of light!

How Does Circadian Rhythm Affect Sleep?

Put simply, your circadian rhythm helps to determine your sleep patterns. That “master clock” in the hypothalamus controls when your body produces the hormone melatonin.

Melatonin is the hormone that causes you to feel sleepy. When your eyes take in less light, like at night, your body produces more melatonin. This is why you may feel drowsy after spending time in a dark room.

This is also why you may have a difficult time falling asleep after spending the evening staring at the bright light from your laptop or cell phone; when your eyes take in more light, your body produces less melatonin.

circadian cycle

Studies have found that melatonin levels can be influenced in particular by the presence of blue light, which is found more in artificial light sources, such as LED bulbs, than in natural sunlight.

How Does Circadian Rhythm Affect Your Health?

Your circadian rhythm can dictate when you fall asleep, when you wake up, and how long you sleep overall. Therefore, when your circadian rhythm becomes disrupted, so does your sleep.

Studies have found that a lack of sleep, and the disruption of circadian rhythms caused by this sleep loss, can contribute to the development of health conditions such as obesity, depression, and many chronic diseases.

If your job requires you to be awake at unusual times, if you travel to a different time zone, or if you take certain kinds of medications, your circadian rhythm can be thrown out of its regular cycle, leading to these and other negative health effects.

Researchers are still trying to understand exactly how our biological clocks and circadian rhythms affect our sleep cycles, to create better treatments for negative health conditions such as sleep disorders, obesity, mental health disorders, and jet lag.

This could also help individuals adjust to jobs that require them to work at night, or that have irregular schedules. Studies conducted to examine circadian rhythms are often performed on mice and fruit flies, which have similar biological clock genes to humans.

In these studies, researchers control the subject’s environment by changing when it is light and dark around them, and then they keep an eye on changes in the animals’ behavior and biology.

Circadian rhythms also affect a multitude of other aspects of our body and health. Asthma is commonly the worst in the early morning, potentially because our biological clocks keep the production of cortisol to a minimum at night, and cortisol is an anti-inflammatory steroid.

Allergies also commonly flare up when you first awaken, with more sneezing and runny noses at this early point in the circadian cycle.

Additionally, our blood pressure tends to be at its lowest around three in the morning, and to rise sharply around dawn; this is why many heart attacks and strokes occur early in the day, between 8 a.m. and noon.

Interestingly, studies have found that humans are more likely to give birth at night than during the day.

This is possibly because, in the distant past, it was more likely for mothers and newborns to survive this vulnerable stage when it was too dark to be found by predators. As such, evolution favored people with hormone cycles that led to nighttime births.

This video goes into more detail on the circadian cycle.

What Influences Circadian Rhythm?

There are many lifestyle factors that can influence your circadian rhythm, such as how and when you sleep, when and what you eat, and how much and when you exercise.

The important thing is to be aware of your personal natural circadian rhythms, so that you can manage your lifestyle in a way that easily supports a healthy sleep cycle.

There is no “one size fits all” way to maintain a healthy circadian rhythm, so first figure out whether you are a “morning person” or a “night person” – that is, at what time of day are you personally happier, more active, and more aware?

Once you figure that out, establish a consistent daily routine. Try to eat, exercise, and go to bed at the same time every day of the week – and yes, that includes weekends!

When your body is used to being active at one time every day, and going to bed at one time every night, without interruptions, then your body is more able to develop a consistent circadian rhythm.

As such, you will find yourself falling asleep more easily on cue, as well as feeling more awake, alert, and active during the day!

Consistently sleeping for the recommended amount of seven to nine hours per night can help you, as an adult, make the most out of your circadian rhythms.

circadian clock

If you must stay up late or get up early, try to keep that change to less than half an hour away from normal, so that your body is able to naturally anticipate when you should be asleep or awake.

As for eating, it is better to eat most of your daily calories early in the day, rather than later. At night, your internal biological clock tends to change the role of your liver. During the day, the liver creates enzymes that convert calories to energy, while, at night, it produces enzymes that store energy.

If you eat a lot right before bed, your liver will end up working overtime, which can contribute to digestive problems that interrupt your sleep. For this reason, the daytime is better for exercise than nighttime as well.

During the day, your body is in “burn calories and create energy” mode, and that is what exercise is all about. In general, working with your body’s rhythm is better than working against it!

Eating foods that are rich in the hormone melatonin can help you adjust your circadian rhythm. Tart cherries, goji berries, walnuts, almonds, tomatoes, oranges, pineapples, and bananas are some foods that contain high melatonin levels, which can help you sleep.

Other things that you eat or drink, such as caffeinated products in the form of coffee and soda, have been found to reduce your melatonin production.

This means that a small amount of coffee in the morning can help you wake up – as you might have already noticed.

But if you drink a lot of it throughout the day and into the afternoon and evening, you can alter your melatonin production enough that you no longer produce enough of this hormone to successfully fall asleep when you need to!

Naps are also a great tool for maintaining your circadian rhythm. A brief nap at just the right moment can increase alertness in the short-term, as long as you work with your circadian rhythm instead of against it.

Find the low point of alertness in your circadian cycle; for most people who work standard “nine in the morning to five at night,” this point is around three in the afternoon.

With the boost from a brief nap, you will be even more alert later on, during the natural high points in your circadian rhythm.

circadian rhythm

Circadian Rhythm and Jet Lag

If you have ever traveled a long distance by plane, then you probably already know about jet lag – the way you have a difficult time adjusting to the new schedule when you reach a different time zone than the one you came from.

When you travel between time zones, your circadian rhythms can remain aligned with the time zone you started out in. This means that your body might expect to be asleep even though it is daytime where you now are, or to be awake when you know you should be going to bed in the new time zone.

Jet lag is, therefore, a circadian rhythm sleep disorder, because your body becomes confused about when to produce melatonin and help you sleep.

Jet lag is, thankfully, a temporary experience. It can be more or less severe depending on how many time zones you crossed and also which direction you were traveling in.

When you fly to the east, where it is “later” in the day than where you started, you may have a more difficult time with jet lag than when you travel to the west, where it is “earlier” in the day than where you started.

Usually, it takes one day per time zone traveled through in order for your body clock to completely adjust to the new local time, though older adults have a harder time with it and may need several more days to recover.

The problem with jet lag is that it makes it difficult for you to function, turning important things like work or sightseeing into difficult tasks.

Pilots, flight attendants, and people who travel frequently for their businesses are likely to experience jet lag often due to their on-the-go lifestyles.

Jet lag can also be made worse by a general loss of sleep or heightened stress due to travel, by spending a lot of time sitting in an uncomfortable position like an airplane seat often creates, and by drinking caffeine or alcohol.

circadian rhythm

When you travel a long distance, be sure to factor in time to recover when possible! A few weeks before your trip, start going to bed earlier or later depending on what direction you are traveling, to get your body closer to the sleep schedule you will use in the new location.

Also, be sure to expose yourself to plenty of sunlight when you arrive at your destination, to help reset your internal clock!

Melatonin supplements, sleeping pills, and moderate exercise are other ways to adjust your circadian rhythms and help yourself get back on track with your new sleep schedule.

This video goes into more detail on jet lag and your circadian rhythm.


Now you have the knowledge to understand your circadian rhythm and how it affects your sleep and your health.

You also have some tools and strategies that you can use to take charge of your lifestyle and become more well-rested and healthier overall!

What is your circadian rhythm schedule like?


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