Your Guide to the Circadian Rhythm

Medically Reviewed By Suzanne Stevens, MD
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Circadian rhythms are the natural cycles of physical, mental, and behavioral changes that your body experiences in a 24-hour period. The National Institute of General Medical Sciences (NIGMS) explains that circadian rhythms are natural processes in the body that include physical, mental, and behavioral changes every 24 hours.

Contrary to popular belief, sleeping is only one circadian rhythm, not the only one. All circadian rhythms respond to light and darkness, and they occur in almost all living creatures. 

This article explains more about circadian rhythms, including how they work and impact health, what affects them, and how to support the body’s circadian rhythms. 

What are circadian rhythms? 

man unable to sleep
Michela Ravasio/Stocksy United

The Centers for Disease Control and Prevention (CDC) notes that there are several systems involved in circadian rhythms, including:

  • sleep and wake cycles
  • hormone release
  • body temperature rhythm
  • eating and digesting

All of the rhythms are reset by the sun’s setting and rising approximately every 24 hours. Some of the processes run longer than 24 hours, according to the CDC, but they are reset every 24 hours by the light-dark influences. 

What is the master clock? 

The CDC also explains that the “master clock” is the master circadian clock that controls all four rhythms and keeps all the body clocks in sync together. 

According to the NIGMS, 20,000 different nerve cells make up the master clock. Put together, those nerve cells form the suprachiasmatic nucleus (SCN). The SCN is in the hypothalamus, which is a small area in the brain that controls hormone release and regulation. 

What is a biological clock? 

The NIGMS also explains that all living organisms, from fungi to fruit flies, have innate biological clocks. These biological clocks regulate the circadian rhythms through processes that involve proteins interacting with different body cells. 

Biological clocks in the body are internally and naturally driven. For instance, certain cell proteins will build up at night and decrease during the day, which influences feelings of being sleepy or awake. 

In most cases, and for optimum health, your biological clock will line up with your circadian rhythms. If your biological clock does not line up with your circadian rhythms, health problems such as diabetes, obesity, and sleep disorders can occur. This is according to the National Sleep Foundation (NSF)

What are examples of circadian rhythms in nature? 

All living organisms have circadian rhythms and biological clocks. For instance, one paper describes how circadian and biological clocks control insects’ metamorphosis into adult versions.

Another paper explains that even flowers have natural circadian rhythms that control them opening up their petals in the morning and closing them at night. As the NSF explains, nocturnal animals also have circadian rhythms that allow them to sleep during the day and be awake at night to hunt for food or be alert to predators. 

How do circadian rhythms work?

Circadian rhythms work through light activation. The master clock activates every day when light hits the eye. This happens even when you are sleeping and your eyelids are still closed. The retina receives the light signal and sends it down a nerve to the master clock in the brain.

From there, the master clock activates the different circadian rhymes and controls. For instance, the light activation will decrease the production of melatonin, which is the hormone that makes us go to sleep, and increase the production of other hormones needed to be awake.

Although many aspects of circadian rhythms are innate, external and environmental factors — such as disrupted sleep or light exposure —

can influence them. 

How do they work for babies, teenagers, and adults?

Circadian rhythms will change at different stages in a human’s life. For instance, babies are not born with set circadian rhythms. Instead, they develop during the first few months of life. 

One paper explains that babies do not even begin to develop their circadian rhythms until around 8 weeks old. Their sleep circadian rhythm will not start to develop until closer to 9 weeks.

The NIGMS also explains that teenagers’ circadian rhythms are also very different from those of adults. Notably, melatonin — which is the hormone primarily responsible for making someone feel tired — increases about 1 hour later than in adults. Additionally, teenagers’ alertness peaks later in the morning than that of adults, and teenagers may not feel fully awake until 9 a.m. or 10 a.m.

As humans age, they shift back into a circadian rhythm that is present in younger children: early to bed and early to rise. 

How does the body keep its own circadian rhythms?

The body keeps its own circadian rhythm through the activation of the “master clock” in the brain. The master clock is primarily activated through exposure to light in the morning, and it runs on a set schedule for around 24 hours after that. 

The internal processes of circadian rhythms are largely innate. However, they do differ among people based on their genetic makeup, their age, and even external factors, such as their sleep environment and lifestyle. 

How do circadian rhythms affect health? 

Disrupted circadian rhythms can be detrimental to overall health. Without the right amount of sleep, for instance, the body will not be able to control other necessary processes, such as hormone production and regulation.

According to the NIGMS, disrupted circadian rhythms can lead to health conditions such as:

Additionally, underlying health conditions can disrupt circadian rhythms. For instance, obstructive sleep apnea can disrupt sleep, and it is also linked with the development of certain neurological disorders, such as Alzheimer’s disease

How do they affect sleep? 

The sleep-wake circadian rhythm is a circadian rhythm that we are very familiar with. The sleep-wake cycle is highly influenced by the light-dark cycle of the sun, so knowing how it is affected can help you sleep better at night. 

The sleep-wake cycle uses “clues” from daytime light and darkness to help prepare your body for what it thinks it needs to do later on. For instance, dimming the lights in your environment or prohibiting exposure to the sun

in the evening primes your body to start feeling sleepy about 2 hours later. 

On the flip side, bright light primes your brain to get ready to wake up and be most alert in approximately 1 hour.

So, to help ensure that you sleep, you should create a quiet and dark environment about 2 hours before you want to go to bed. You may also wish to expose yourself to natural light or bright light about 1 hour before you need to be fully awake. 

Get more tips for better sleep health here.

When should you contact a doctor? 

You should always contact a doctor for any new changes to your health or behavior.

In general, for sleep-related concerns, you should schedule a checkup with your doctor if you notice that you:

  • are falling asleep at unusual times, such as in the middle of the day or very early in the evening
  • feel perpetually tired
  • are finding it difficult to fall asleep at night
  • have sleep behaviors that are different than usual
  • do not feel rested, even with healthy sleep habits and a consistent bedtime
  • have a health condition and are having difficulty sleeping
  • wake up frequently and cannot go back to sleep

Some people rely on taking melatonin supplements to help them sleep, but the CDC cautions that using melatonin incorrectly can actually disrupt natural sleep-wake cycles. You should always consult with your doctor if you are considering taking melatonin. 

What can disrupt circadian rhythms?

The NIGMS explains that both internal and external factors can affect and potentially disrupt circadian rhythms. These factors include: 

  • genetic mutations 
  • jet lag
  • shift work
  • electronic devices 
  • melatonin use, especially at the wrong time for your sleep cycle
  • light exposure at night
  • sleep disorders 
  • caffeine consumption
  • disrupted sleep-wake patterns, such as being woken up at night
  • illness and infection
  • other medical conditions
  • stress and anxiety
  • certain medications

Some of these factors are out of our control, such as genetic mutations and certain medical conditions, while others may be able to be controlled, such as avoiding screens at night. 

How can you maintain healthy circadian rhythms? 

Again, not all factors that disrupt the circadian rhythm are controllable. For instance, shift workers may not be able to quit their jobs, and people who have to travel for work may not be able to change those responsibilities right away.

However, there are some things that you can do to support and maintain healthy circadian rhythms. These include:

  • exposing yourself to natural light as much as possible during the day
  • trying to maintain a consistent sleep schedule
  • going to bed and getting up around the same times every day
  • avoiding caffeine consumption late in the day
  • exercising daily, but not too close to your bedtime
  • avoiding large meals right before going to bed
  • avoiding bright lights and screens 2 hours before bed
  • keeping lights dim at least 2 hours before bed
  • avoiding late-day napping
  • making your sleep area a calming one, with minimal clutter and distractions

The CDC also suggests using the light-dark aspects of circadian rhythms to help you create a more consistent sleep schedule, especially if you are struggling with sleep. They note that you can try:

  • wearing dark sunglasses 2 hours before bed if you cannot control the light in your environment
  • using bright lights to combat sleepiness if it is too early for bed 
  • keeping lights dim if you wake up too early or in the middle of the night
  • turning lights on or letting natural light in if you are sleepy in the morning

Summary

Circadian rhythms are natural processes in the body that control things such as hormone regulation, sleep and wake cycles, body temperature, and eating and digesting food. They are primarily controlled by the “master clock” in the body that activates by natural light hitting the eye, even if the eye is shut. 

The circadian rhythms operate on a roughly 24-hour cycle and reset every day when the master clock activates. However, external factors — such as travel, stress, caffeine consumption, artificial lights from screens, and shift work — can all influence circadian rhythms as well.

Additionally, circadian rhythms change as we age. Babies do not develop them until closer to 3–4 months old, teenagers have different rhythms that make them want to stay up later and sleep in later, and older adults like to sleep earlier and get up earlier. 

Circadian rhythm disruptions can be dangerous and lead to health problems such as diabetes and obesity. Additionally, certain health conditions can cause disruptions to circadian rhythms, so optimizing circadian rhythms is important to overall health. 

You should contact a doctor if you are having excessive daytime sleepiness, having difficulty sleeping at night, waking up a lot at night, or considering starting melatonin.

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Medical Reviewer: Suzanne Stevens, MD
Last Review Date: 2022 May 25
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