What Is Circadian Rhythm and How Does It Impact Sleep?
Written by: Gabrielle Talan
Gabrielle, a former insomniac turned sleep writer, brings over a decade of content expertise to her work. For the past 3 years, she's focused exclusively on sleep topics – a happy choice that helps give her the deep and restful sleep she craves.
Ever wondered why you can't just fall asleep whenever you want? Or why is it that jet lag turns your sleep schedule upside down for days? Maybe you're struggling with insomnia and can't figure out why you have such a hard time drifting off and staying asleep.
The answer to these sleep issues comes down to two words: circadian rhythm. Your circadian rhythm is your body's internal 24-hour clock that tells you when to feel sleepy and when to be awake. When it's out of sync, deep and restful sleep can be elusive.
This article will walk you through what the circadian rhythm is, how it impacts sleep, what can throw it out of whack and how to keep it healthy.
Key Takeaways
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The circadian rhythm is the body's 24-hour internal clock that regulates sleep and wakefulness, hormone production, metabolism, mood, alertness, cognitive performance and body temperature.
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A misaligned circadian rhythm affects both short- and long-term health and overall well-being.
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You can maintain a healthy circadian rhythm with good sleep hygiene and consistent, healthy daily habits.
Table of Contents
What Is Circadian Rhythm?
The circadian rhythm is your body's internal 24-hour clock that regulates when you feel sleepy or awake. Apart from influencing your sleep-wake cycle, it also affects the following:
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Metabolism and digestion - It influences the efficiency of your digestive system.
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Hormone production - It controls hormone release: melatonin at night for sleep, cortisol in the morning for alertness, the growth hormone for tissue repair, plus thyroid and reproductive hormones.
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Mood - It also regulates serotonin (the happy hormone) and other neurotransmitters to keep your mood and emotions stable.
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Alertness - It creates natural highs and lows in alertness throughout the day, driven by the release of cortisol and melatonin. Energy is usually highest in the late morning and early to mid-evening. It's lowest in the early afternoon and late at night.
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Cognitive performance - It controls when your brain is at its peak. Problem-solving, memory and attention are at their highest in the late morning and dip in the early afternoon.
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Body temperature - It controls body temperature in a daily pattern. Core body temperature fluctuates at approximately 1 to 2 degrees Fahrenheit over 24 hours. For the average sleep schedule, it drops to its lowest point at around 4 to 6 a.m. for sleep and peaks between 4 and 7 p.m. for alertness.
Now that you know what the circadian rhythm influences, let's explore how it works.
How Do Circadian Rhythms Work?
Circadian rhythms are controlled by a master clock known as the suprachiasmatic nucleus (SCN) located on top of where your optic nerves cross.
The SCN follows a cycle that's longer than 24 hours. According to The Harvard Gazette, the average adult's internal body clock is about 24 hours and 11 minutes, though it varies from person to person.(1) To maintain alignment with the earth, circadian rhythms need to adjust by about 11 minutes per day by relying on environmental cues, also known as zeitgebers, the most important of which is the earth's natural light-dark cycle.
Other zeitgebers include exercise, meals, stress and daily routines. Essentially, you can control when you sleep and when you wake up by timing these appropriately and consistently.
Light is key. When your retinas see light, especially blue light, they signal your SCN that it's daytime, triggering melatonin suppression, cortisol release, a rise in body temperature and metabolism. Darkness triggers the opposite.
Sleep changes with age, and circadian rhythms are no exception.
Newborns follow an ultradian rhythm, a shorter, repetitive sleep cycle of about 2 to 4 hours, resulting in frequent waking. They start developing a circadian rhythm at about 6 to 8 weeks. By their first year, their circadian rhythm is stronger and more stable.
Kids' circadian rhythms are set earlier than those of adults, leading to bedtime and nap resistance, earlier wake-up times, and nighttime awakenings. In their teens, the circadian rhythm shifts to later (known as sleep phase delay). Teens tend to go to bed later and wake up later.
Once a person reaches adulthood, their circadian rhythms stabilize. However, individual chronotypes still vary. Working with your body's natural rhythm can help you get sufficient rest and perform at your best during your day-to-day.
How Does Circadian Rhythm Impact Sleep?
Circadian rhythm is the gatekeeper of your sleep. It regulates your sleep-wake cycle by releasing hormones at specific times throughout the day and night. It's what makes you feel sleepy or awake.
The circadian rhythm takes darkness as its signal to trigger the release of melatonin, the sleep-regulating hormone. It peaks at night while you're asleep and declines in the morning, when you're ready to wake up. At the same time melatonin rises, your body temperature drops, which is necessary for deep and restful sleep.
The circadian rhythm signals to your body that it's time to wake up by suppressing melatonin production and using light as a cue to release cortisol, helping you feel alert. Your body temperature increases, which also boosts alertness levels.
Your circadian rhythm needs to be in sync with your sleep-wake schedule so that waking up and falling asleep is easy and natural. You can do so by maintaining a consistent sleep-wake schedule, getting natural light exposure during the day and turning down the lights so that darkness can do its job in the evening.
What Happens When Your Circadian Rhythm Is Out of Sync?
When your body's internal clock is misaligned, you might start having sleep problems, such as insomnia. You might feel tired but can't sleep or wake up multiple times throughout the night, causing sleep deprivation. You might also experience sleep inertia – feeling groggy and disoriented when you get up in the morning.
Disrupted circadian rhythms can put you in a bad mood because the release of mood-regulating neurotransmitters is also thrown off. You might feel irritable, depressed or anxious.
The circadian rhythm also controls when your cognitive performance is at its peak. It can affect your memory, focus and problem-solving abilities when it's out of sync.
A misaligned circadian rhythm can also cause you to gain weight. Your metabolism slows, and your food cravings heighten, leading to overeating. It could also lead to digestive issues.
Now, let's talk about the long-term effects. Chronic circadian rhythm disruption can lead to a host of health problems, such as diabetes, heart disease, obesity and some cancers. A paper published in Cell Death Discovery found that when your circadian rhythm is consistently disrupted, it can also alter how certain genes behave, including those involved with cancer prevention. It also weakens your immune system, making it less able to fight illness.(2)
You might also develop circadian rhythm sleep disorders, such as shift work disorder (SWD) and advanced sleep phase disorder. Shift work disorder affects exactly who its name suggests – shift workers who work the night shift or non-traditional hours. It's characterized by insomnia and extreme daytime sleepiness. Meanwhile, advanced sleep phase disorder is when your internal clock is set earlier than usual. Symptoms include feeling sleepy very early in the evening and waking up earlier than normal in the morning.
Things That Can Disrupt Your Circadian Rhythm
Your circadian rhythm can get thrown off by several things, including:
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An irregular sleep schedule - Going to bed and waking up at the same time every day tells your circadian rhythm when to start releasing melatonin for sleep or cortisol for wakefulness. An irregular sleep schedule confuses it and prevents it from adapting to any sleep pattern.
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A bedroom that isn't optimized for sleep - Light, darkness and temperature are zeitgebers for your circadian rhythm. Darkness triggers melatonin release and a necessary drop in body temperature, allowing your body to transition into sleep. While it doesn't directly affect your circadian rhythm, noise can also disrupt sleep.
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Exposure to light at the wrong time - Light exposure in the evening confuses your circadian rhythm into thinking it's daytime. It suppresses melatonin, too.
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Shift work or jet lag - These can misalign your circadian rhythm by flipping your sleep schedule. Shift work keeps your circadian rhythm constantly at odds with your schedule. It forces you to stay awake when your body is naturally winding down, and you feel sleepy when your body is supposed to be awake. Jet lag, while temporary, disrupts your circadian rhythm as your body crosses multiple time zones. When you reach your destination, the local time is misaligned with your circadian rhythm, which is still operating on your home time zone.
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Stress - Your circadian rhythm controls the release of cortisol, following a specific pattern. Stress triggers additional cortisol release. Chronic stress causes your body to release cortisol even when it should be reducing it. On the other hand, melatonin release is suppressed. You'll struggle with sleep because your brain stays alert even when it's supposed to be preparing for bed.
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Eating late at night - The circadian rhythm also regulates digestion and metabolism, so it expects you to eat during the day. Eating late at night confuses it, causing your body to focus on digesting and metabolizing rather than resting and repairing.
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Certain medications - Some beta blockers, corticosteroids, stimulants, diuretics, decongestants and antidepressants can disrupt your circadian rhythm. They do so by interfering with the hormones and neurotransmitters your circadian rhythm depends on, as well as your sleep-wake patterns.
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Underlying health conditions - Insomnia, depression, Alzheimer's disease, thyroid disorders, fibromyalgia, heart disease and GERD are some underlying health conditions that can disrupt your circadian rhythm. They can severely disrupt your sleep quality, directly damage your circadian rhythm and interfere with the hormones and neurotransmitters that underpin it.
How to Maintain a Healthy Circadian Rhythm?
Understanding how to maintain a healthy circadian rhythm is super important. Putting an end to poor sleep habits with good sleep hygiene and a healthy lifestyle is key. Here are a few tips:
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Maintain a consistent sleeping and eating schedule - Sleep and wake up at the same time every day. That said, eating at the same time every day will also help – remember that meal time is another zeitgeber for your circadian rhythm.
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Stick to a relaxing bedtime routine - Banish stress and anxiety with relaxing pre-bedtime activities, such as meditation, yoga or reading a book.
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Keep your bedroom cool, dark and quiet - Make sure your bedroom has an ideal sleeping temperature between 60 and 67 degrees Fahrenheit. Use blackout curtains or a 100% light-blocking sleep mask to achieve complete darkness. Block out noisy neighbors or your partner's snoring with earplugs or mask it with a white noise machine.
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Exercise regularly - Engage in physical activity every day. You should stick to a schedule for this, too, because your circadian rhythm will also rely on it as one of its cues.
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Avoid napping too late in the day - Napping is healthy. However, don't do it too often or too late in the day. Otherwise, you'll struggle with sleep at night.
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Stop looking at screens before bedtime - Light cues the circadian rhythm to send wakefulness signals to the brain, including artificial light from gadget screens. Put away your gadgets at least an hour before going to bed.
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Expose yourself to light in the morning - Step outside and soak in the bright light from the sun when you get out of bed. You should also step outside if you start to feel groggy and tired in the afternoon. Shift workers can benefit from bright light therapy.
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Steer clear of caffeine, nicotine and alcohol in the evening - Caffeine and nicotine are stimulants that'll keep you awake. Alcohol might help you fall asleep faster, but it can cause you to wake up often throughout the night.
Conclusion
Think of your circadian rhythm as your body's timekeeper that runs on a 24-hour cycle. It tells your body when to sleep and when to be awake. It also regulates other biological processes, such as hormone production, metabolism and alertness.
When it's aligned with your sleep-wake cycle, you'll fall asleep faster and sleep better. You'll also feel more refreshed and alert during the day. The opposite happens when your circadian rhythm gets thrown off.
The great news is you can control your circadian rhythm by maintaining good sleep hygiene and a healthy lifestyle. If you still experience sleeplessness despite doing so, consult a sleep specialist.
Frequently Asked Questions About Circadian Rhythm
Is Circadian Rhythm the Same Thing as a Biological Clock?
No, circadian rhythm isn't the same as a biological clock. However, the two are related. The biological clock (also known as the circadian clock or master clock) is the suprachiasmatic nucleus (SCN) that creates 24-hour cycles, known as circadian rhythms. Think of the biological clock as your body's timekeeper, and circadian rhythms as schedules that follow it.
How Do I Know if My Circadian Rhythm Is Off?
You'll know your circadian rhythm is off when you start experiencing sleep issues like insomnia, and circadian rhythm disorders like shift work disorder and jet lag disorder. You might also experience mood issues, weight gain, decreased cognitive performance or a weakened immune system.
Why Do I Feel Sleepy at Certain Times of the Day, Even if I Slept Enough?
You feel sleepy at certain times of the day, even if you slept enough, because your circadian rhythm causes a natural dip in your energy and alertness levels.
This is most commonly felt in the early afternoon, between 1 and 3 p.m. You'll also feel a dip late in the evening, when your circadian rhythm increases melatonin production and lowers your core body temperature, preparing you for sleep.
You might also feel sleepier as the day wears on, as your circadian rhythm works alongside adenosine, a chemical that accumulates in your brain, building sleep pressure.
Can Circadian Rhythm Disruptions Cause Insomnia?
Yes, circadian rhythm disruptions can cause insomnia. When your circadian rhythm is misaligned with your sleep-wake schedule, your body gets confused about when to be asleep or awake. You might be unable to sleep at night because melatonin production is suppressed, and your body temperature remains high. You might also wake up often throughout the night because your circadian rhythm isn't controlling your sleep cycles, preventing you from progressing to the deep sleep stages.
Does Daylight Saving Impact Circadian Rhythm?
Yes, daylight saving time can impact circadian rhythm because it involves a 1-hour shift, and your internal clock depends on consistency. It doesn't instantly change and requires time to adjust.
Essentially, you're waking up and sleeping an hour earlier or later than what your circadian rhythm is accustomed to. You might wake up feeling groggy and irritable, and have difficulty sleeping until your circadian rhythm adjusts to the new sleep-wake schedule, which usually takes about a week.
Is Napping Bad for My Circadian Rhythm?
No, napping isn't bad for your circadian rhythm when done right. Napping in the early afternoon, between 1 and 3 p.m., is advisable because it aligns with your natural rhythm. Your circadian rhythm causes your energy levels to slump at this time, so your body benefits from a quick 20- to 30-minute nap. However, napping for too long after 3 p.m. can deplete your sleep pressure, causing sleeplessness at night. It can also cause grogginess and disorientation that last for 30 minutes to an hour if you wake up in the deep sleep stage.
Does Caffeine Affect Circadian Rhythm?
Yes, caffeine affects your circadian rhythm in two ways when consumed after 2 or 3 p.m. First, it blocks adenosine receptors in your brain, preventing the chemical that makes you feel sleepier throughout the day from doing its job. It is also a stimulant that impacts your internal clock's ability to help you wind down for bed. It might even push your circadian rhythm later, making it difficult to wake up when you need to.
Sources:
(1) "Human biological clock set back an hour.” The Harvard Gazette, news.harvard.edu/gazette/story/1999/07/human-biological-clock-set-back-an-hour/. Accessed 27 February 2026.
(2) “Circadian rhythm regulates the function of immune cells and participates in the development of tumors.” Cell Death Discovery, pubmed.ncbi.nlm.nih.gov/38678017/. Accessed 27 February 2026.
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