A circadian rhythm is a physical, mental or behavioral change in your body that repeats on a cycle of about 24 hours. A clock in your brain keeps the time, and clocks in your organs follow it. Light entering your eyes resets that clock every day. It sets when you sleep and wake, when your core temperature falls, and when your body releases melatonin and cortisol.
- A circadian rhythm is any body process that comes around on a cycle of roughly 24 hours. The National Institute of General Medical Sciences names sleep, hormone release, appetite, digestion and body temperature among them.
- The clock does not run at exactly 24 hours. Czeisler and colleagues, writing in Science in 1999, measured the average human period at 24.18 hours, which is why daylight has to correct it every day.
- Jeffrey C. Hall, Michael Rosbash and Michael W. Young shared the 2017 Nobel Prize in Physiology or Medicine for working out the molecular machinery that keeps that time.
- A circadian rhythm is any body process that repeats about every 24 hours.
- One clock keeps the time and every organ keeps its own.
- The clock runs close to 24 hours, and light closes the gap.
- Your temperature, your hormones and your hunger keep the same timetable.
- The clock's gears were found in fruit flies.
- Your chronotype is where your clock sits, not how disciplined you are.
- The clock you are born with is not the one you keep.
- When the rhythm drifts, you notice it as something else.
- What a bed can do about your rhythm, and what it cannot.
- Where to start tonight.
- How we checked
- FAQ
A circadian rhythm is any body process that repeats about every 24 hours.
What is a circadian rhythm? It is a change in your body that comes around on a cycle of roughly one day. The National Institute of General Medical Sciences defines circadian rhythms as the physical, mental and behavioral changes an organism goes through over a 24-hour cycle. The name is Latin: circa means around and diem means day.
That definition is wider than sleep. Your core temperature rises and falls on a rhythm. So do melatonin and cortisol, your appetite, your digestion and how sharp you feel at four in the afternoon. Sleep is the rhythm you notice, and it is one of a crowd.
The words below get used as though they meant one thing. Most muddled sleep advice comes from mixing them up.
| Term | What it means | How long one lasts | Where you meet it |
|---|---|---|---|
| Circadian rhythm | A body process that repeats on a cycle of about 24 hours | About a day | Sleep timing, temperature, hormones, appetite |
| Biological clock | The machinery that keeps the time, built from genes and the proteins they make | Runs without stopping | Nearly every cell you have |
| Master clock | The group of nerve cells that holds every other clock in step | Runs without stopping | The suprachiasmatic nucleus, in the brain |
| Sleep cycle | One pass through light sleep, deep sleep and dreaming sleep | About 90 minutes | Four or five times a night |
| Jet lag | A short mismatch between your clock and local time after you fly | Days | After crossing time zones |
| Social jet lag | The same mismatch produced by a work schedule instead of a flight | Ongoing | Weekday alarms, weekend lie-ins |
| Circadian rhythm sleep-wake disorder | A clinical mismatch between your clock and the day you have to live | Months to years | Diagnosed by a clinician |
Method: the rows above follow the National Institute of General Medical Sciences fact sheet and the Sleep Foundation's published pages. Nothing here was measured by us.
Photo: Circadian
One clock keeps the time and every organ keeps its own.
Where does a circadian rhythm come from? A master clock in your brain, sitting just above the point where the nerves from your eyes cross. StatPearls, on the National Library of Medicine's bookshelf, describes about 20,000 densely packed nerve cells in a structure called the suprachiasmatic nucleus, about the size of a grain of rice.
That clock is not alone. Your liver keeps one. So do your lungs, your heart, your gut, your pancreas and your skeletal muscle. A 2025 review in Frontiers in Endocrinology lists all of those organs running a daily cycle of gene activity of their own.
The brain clock does not do their work for them. It sends the time, through nerves and hormones, and each organ runs its own schedule against it. So the system can come apart in pieces. Eat in the small hours for a month and your gut can end up on a different schedule from your brain.
Recommended Reading
How Light Sets Your Circadian RhythmWhat morning light does, what evening light undoes, and how much it takes to move the clock.
The clock runs close to 24 hours, and light closes the gap.
Why do people say about 24 hours instead of 24? Your clock does not keep perfect time. Czeisler and colleagues measured it in people living for weeks under controlled lighting, and reported an average period of 24.18 hours in Science in 1999. Call it eleven minutes a day.
Eleven minutes sounds like nothing. Left alone it is more than an hour a week, and inside two months your body would be keeping night while the sun was up. Light is the correction.
Light lands on your retina and travels to the master clock. The NIGMS fact sheet describes that clock setting melatonin production by how much light your eyes take in, raising it in the evening and holding it down in daylight. Morning light pulls your clock earlier, and evening light pushes it later. Nothing else you do comes close for strength.
Meals, exercise and temperature nudge it too, and they work best when they agree with the daylight. Our guide to resetting a circadian rhythm puts the moves in order.
Your temperature, your hormones and your hunger keep the same timetable.
What does a circadian rhythm control? More than the hour you fall asleep. Five outputs cover most of what you feel in a day.
Sleep, wake and alertness
The clock decides when sleep pressure meets a body ready to take it. How tired you are is a separate system, built on how long you have been awake, and the two have to agree for a night to go well. The same clock puts a dip in your early afternoon and a second wind in the evening.
Core body temperature
Your core temperature falls in the evening, bottoms out in the early hours and climbs before you wake. Your body makes the fall happen by opening the blood vessels in your hands and feet so heat can leave through the skin. A 2011 review in Pflugers Arch reports that the gap between skin temperature at a far site and a near one tracks sleepiness better than any single reading.
Melatonin
Melatonin tells your body it is dark. It does not sedate you. The rise starts a couple of hours before your usual bedtime and holds through the night, and bright light in the evening pushes it back down. All the advice about screens rests on those two facts.
Cortisol
Cortisol runs the other way. It sits low through the night, climbs before you wake, and peaks 30 to 60 minutes after you open your eyes, according to a 2022 paper in Frontiers in Neuroscience. That morning climb is why you can feel wired after a bad night.
Digestion and appetite
The NIGMS fact sheet lists appetite and digestion among the functions the clock sets. Your gut handles the same meal differently at breakfast and at bedtime, because it keeps a schedule of its own.
The clock's gears were found in fruit flies.
How does a clock made of cells keep time? Fruit flies gave up the answer. The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey C. Hall, Michael Rosbash and Michael W. Young, in the committee's words, "for their discoveries of molecular mechanisms controlling the circadian rhythm."
The machine works like this. A gene called period makes a protein. The protein builds up in the cell overnight, and once there is enough of it, it switches its own gene off. Then it breaks down, the gene starts again, and one loop takes about a day. Young found a second gene, timeless, that lets the protein reach the nucleus. A 2018 review in the Journal of Biomedical Science traces the same loop.
Why should someone buying bedding care? Because it settles what kind of thing a rhythm is. A loop of genes and proteins runs in nearly every cell you own, whether you respect it or not.
Recommended Reading
What Happens to Your Body Temperature During Sleep?The nightly fall in core temperature, where the heat goes, and why the bed is the part you can change.
Your chronotype is where your clock sits, not how disciplined you are.
What is a chronotype? It names where your own rhythm sits against the clock on the wall. Two people can both sleep eight hours and want to start them three hours apart. Neither one is being lazy or virtuous about it.
Till Roenneberg's team at LMU Munich built the Munich ChronoType Questionnaire to measure it, taking the middle of your sleep on free days as the marker. Their 2019 review in Current Biology reports close to 300,000 responses to the online version before it closed in 2017. Chronotype spreads across a population the way height does, with real ends on both sides.
The same group named the cost of ignoring it. Social jet lag is the gap between the sleep your clock wants and the sleep your Monday allows, and late chronotypes carry the most of it. The alarm does the work a time zone usually does.
The clock you are born with is not the one you keep.
Does a circadian rhythm change with age? It changes across your whole life, in a direction you can predict. The 2023 review Circle(s) of Life, in the International Journal of Molecular Sciences, traces the arc from before birth to old age.
A newborn arrives without one. Before birth the rhythm comes from the mother, carried across the placenta, and afterward the baby's own clock has to start up. Rhythms in sleep and waking, in temperature and in hormones arrive over the first one to three months. That is why a two-week-old has no sense of night.
Young children run early, waking early and wanting bed early. Then puberty pushes the whole thing later. The same review describes a rapid delay in chronotype that begins with puberty and reaches its latest point around age 20, girls about a year and a half before boys. Crowley and colleagues cover what that costs a school day in the Journal of Adolescence in 2018, and we go through it in our piece on the teenage circadian rhythm.
After twenty the drift reverses, slowly, for decades. By old age that review puts chronotype earlier than it was in childhood, and the strength of the rhythm fades as well. The signal gets quieter, so the body's own gap between day and night shrinks. Early waking in later life is the clock doing what clocks do.
When the rhythm drifts, you notice it as something else.
What happens when your circadian rhythm is off? You rarely feel out of rhythm. You feel wide awake hours after you meant to be asleep, flattened by mid-morning, hungry at odd times and foggy through the afternoon. The mismatch arrives as separate complaints.
Three versions of one problem cover most people. Jet lag moves the world and leaves your clock behind for a few days. Shift work asks you to be awake at the hour your body saves for its lowest temperature. Social jet lag shifts your clock the same way, far more often, through a weekday alarm and a weekend lie-in.
Some of it goes further than a schedule. Circadian rhythm sleep-wake disorders are a real clinical category, and the Sleep Foundation sets out the main ones, from delayed and advanced sleep-wake phase to the irregular and non-24-hour patterns. If your sleep timing has been wrong for months, or it is costing you work or safety, that is a clinician's question. Our guide to circadian rhythm disorders is a place to start, and a doctor is the next step.
Four things people get wrong about the body clock
The first is treating light as a minor factor, when it is the strongest signal your clock takes. The second is chasing the bedtime instead of the wake time, when a fixed wake time with morning light anchors the rhythm. The third is buying a product that claims to reset a rhythm, because none does. The fourth is blaming willpower for a chronotype, when the answer is scheduling and daylight.
Photo: Circadian
What a bed can do about your rhythm, and what it cannot.
So why does a bedding company have an opinion about circadian biology? One output of the rhythm happens in your bed every night. Core temperature falls, and the heat leaves through the skin of your hands, your feet and your face. Bedding decides whether that heat gets out or stays with you.
The claim stops there. No pillow changes, fixes or resets a circadian rhythm, whatever a label says. What bedding can do is keep out of the way of one output. Okamoto-Mizuno and Mizuno's 2012 review in the Journal of Physiological Anthropology reports that under normal covers, heat costs more sleep than cold does, in young adults and older ones alike. The room counts alongside it, and the Sleep Foundation puts the comfortable band at 65 to 68 degrees Fahrenheit.
Your head is a small part of your skin and a large part of the problem, because your face is on the surface all night with no covers to throw off. Wool works on moisture. It takes up to 30 percent of its own weight in moisture vapor before the surface reads wet, so the cloth at your cheek stays cool instead of turning into the warm patch you flip away from. That is why the Organic Wool Pillow suits people who wake up hot.
Choose wool if:
- You wake up hot and find the pillow warmer than the room
- You want springy, medium height rather than firm
- You want the wool and the cover certified together
For the sleeper who wakes up hot
Organic Wool Pillow
Wool takes up to 30 percent of its weight in moisture vapor before the surface reads wet, so your face never lands on the warm patch. The finished pillow is GOTS Certified Organic under license GOTS-10229 from Oregon Tilth.
Shop the Organic Wool PillowHandmade in New Jersey · Free shipping
Cotton reaches the same place by another route, for people who want a steady height. The fiber is dense and the structure around it stays open, so air keeps passing through the pillow year after year instead of sealing into a warm dent. The cotton is grown in Texas, and the finished Organic Cotton Pillow is certified organic under Global Organic Textile Standard license GOTS-10229, cotton inside and cover together. GOTS is the organic standard that covers a finished textile rather than the fiber alone.
Choose cotton if:
- You want a firm pillow that holds its height for years
- You want a plant fiber rather than an animal one
- You want the inside and the cover certified together
Firm, steady, and open to the air
Organic Cotton Pillow
Dense Texas cotton in an open structure, so the pillow holds its height for years and keeps passing air instead of warming into a dent. The finished pillow is certified organic under Global Organic Textile Standard license GOTS-10229 from Oregon Tilth.
Shop the Organic Cotton PillowHandmade in New Jersey · Free shipping
The comforter is the bigger lever, because it covers far more of you. An organic wool comforter keeps heat moving off you at the scale of the whole bed, and it is a duvet insert, so it slips inside a cover and clips at the corners. If you are not sure which pillow suits you, the pillow quiz asks about heat and sleeping position first.
Three people, one question, three answers
A nurse on permanent nights. Her clock is fighting her schedule, and no bed settles that argument. Blackout curtains, a fixed sleep window and daylight at the right end of it do the real work. Cooler bedding makes the sleep she gets less broken.
A teenager who cannot fall asleep before one in the morning. That is puberty moving his clock, not defiance. Morning light and a steady wake time are the levers, and the bedroom should be cool enough that a body dropping its temperature is not fighting the room.
A woman in her fifties waking in the small hours. Heat arriving in waves is usually hormonal, and the first call is a clinician rather than a store. Bedding decides whether each wave clears in minutes or sits under the covers with her. Our piece on waking in the early hours takes the causes one at a time.
A pattern Circadian sees in customer conversations
People write in asking which pillow will fix their sleep schedule, and no pillow does. Four messages later the useful facts arrive: a bedroom at 73 degrees, a foam mattress, a phone in the dark and an alarm that moves two hours on Saturday. The pillow was the cheapest thing on that list, so it got asked about first. Tell us the room temperature and your wake time instead.
Where to start tonight.
So what do you do with all this? Take the two free levers first. Get daylight into your eyes soon after you wake, keep the hour before bed dim, and hold your wake time steady, weekends included.
After that, look at whether the bed lets your body do its part. Core temperature has to fall. A cool room helps, and the best temperature for sleep covers the settings. Natural fiber at the layers touching you helps too, which is why our hot sleeper collection exists and why getting hot at night is our most read piece.
Our wool and cotton pillows are certified organic under Global Organic Textile Standard license GOTS-10229, issued by Oregon Tilth, and the certifications page sets out which product carries what. Ask me for the certificate and I will send it. If you want the pillow question settled, which pillow stays cooler, wool or cotton runs these two against each other.
How we checked
Every number here carries its source in the same sentence, and each source is linked. The definition comes from the National Institute of General Medical Sciences fact sheet. The master clock's size and location come from the StatPearls chapter on the suprachiasmatic nucleus, and the organs with clocks of their own from a 2025 review in Frontiers in Endocrinology. The 24.18 hour period is Czeisler and colleagues in Science, 1999.
Cortisol timing is a 2022 paper in Frontiers in Neuroscience, skin temperature a 2011 review in Pflugers Arch, and heat against sleep is Okamoto-Mizuno and Mizuno in the Journal of Physiological Anthropology, 2012. Chronotype is Roenneberg and colleagues in Current Biology, 2019, and the life course a 2023 review in the International Journal of Molecular Sciences. The Nobel citation is the committee's 2017 press release, and the 65 to 68 degree room and the disorder categories are the Sleep Foundation's pages. The material descriptions are the published structure of each fiber plus our own product pages, and nothing here was measured in a lab by us.
Which pillow fill runs coolest for you?
Six fills, all natural, all adjustable through a side zipper, all filled and sewn by hand in New Jersey. What changes is how each one moves heat and air away from your head, and how firm it feels while it does it.
See all six pillowsThousands of open channels between the hulls carry warm air up and out all night.
Firm and moldable
About 8 pounds
Hulls grown in the USA
Takes on moisture vapor to about 30 percent of its weight, so it never becomes the warm patch you turn away from.
Springy, medium height
GOTS Certified Organic as a finished pillow
From New Zealand flocks
A hollow fiber that is 80 to 90 percent air, so air travels through the whole pillow.
Softest and lightest
About 2 pounds
Vegan, from Indonesia
Open cells and mold pinholes let air pass through every shredded piece.
Bouncy, responsive lift
OEKO-TEX Standard 100 Class 1
Sap from Sri Lanka
Two chambers in one cover. Buckwheat on one side, organic wool on the other, and you flip it.
Firm side and soft side
About 10 pounds
One zipper reaches both chambers
Dense fiber in an open structure, so it holds its shape for years and keeps passing air.
Firm and familiar
GOTS Certified Organic as a finished pillow
Grown on Texas farms
| Attribute | USA-Grown Buckwheat | Organic Wool | Wild-Harvested Kapok. | Tree-Tapped Latex | Buckwool Hybrid | Organic Cotton. |
|---|---|---|---|---|---|---|
| Feel | Firm, moldable, and it holds the shape you push into it | Springy, medium height | Soft, light and down-like. | Bouncy, with a responsive lift | Firm on one side and soft on the other | Dense, firm and familiar. |
| How heat leaves it | Open channels between the hulls carry warm air up and out | Moisture vapor moves into the fiber, so the surface stays cool | A hollow fiber that is 80 to 90 percent air by volume, so it stays cool and soft at once. | Open cells plus the pinholes left by the mold | Both mechanisms, one on each side | Dense fiber in an open structure that keeps passing air. |
| Weight, and what it buys | About 8 pounds, heavy enough that it stays put | Medium, soft enough to move around at night | About 2 pounds, the lightest and softest in the range. | Medium, and it holds your head where you put it | About 10 pounds, which steadies the buckwheat side | 3 to 4 pounds, firm and steady for years. |
| Height set by you | Yes, through the side zipper | Yes, through the side zipper | Yes, through the side zipper. | Yes, through the side zipper | Yes, one zipper reaches both chambers | Yes, through the side zipper. |
| Cover | Organic cotton twill, woven in the USA | Organic cotton, woven in the USA | Organic cotton, woven in the USA. | Organic cotton, woven in the USA | Organic cotton twill, woven in the USA | Organic cotton, woven in the USA. |
| Certification | Organic cotton cover, and the fill inside is USA-grown, cleaned with air and UV light rather than a chemical finish that could off-gas. | GOTS Certified Organic as a finished pillow, fill and cover together, license GOTS-10229, Oregon Tilth. | Organic cotton cover, and the fiber inside is gathered wild from seed pods. | The latex inside is certified to OEKO-TEX Standard 100 at Class 1, with FSC certified rubber-tree sourcing. | Organic wool on one side, inside an organic cotton cover. | GOTS Certified Organic as a finished pillow, fill and cover together, license GOTS-10229, Oregon Tilth. |
| Fiber origin | Regional farms in the USA | Certified organic farms in New Zealand | Ceiba pentandra pods in Indonesia. | Hevea brasiliensis plantations in Sri Lanka | Hulls from the USA and wool from New Zealand | Farms in Texas, USA. |
| Vegan | Yes, it is plant matter | No, wool comes from sheep | Yes, a plant fiber. | Yes, it is tree sap | No, one side is wool | Yes, a plant fiber. |
| Sound | A soft rustle, quieter after pre-polishing | Silent under your head | Silent under your head. | Silent under your head | A rustle on one side, silence on the other | Silent under your head. |
| Best for | The coolest surface and the firmest neck position | Waking up hot, and heat that arrives in waves | Down softness without the feathers. | A springy lift that does not sink | Wanting firm and soft in one pillow | Firm, steady height that lasts for years. |
| Sleep position | Side and back | Any, because you set the height | Back and front. | Side and back | Side and back | Side and back. |
| Care | Wash the cover and air the hulls | Wash the cover and air the wool in indirect sun | Wash the cover and air the fiber. | Wash the cover and air the pieces | Wash the cover and air each chamber | Wash the cover and air the cotton. |
| Where made | Filled and sewn by hand in New Jersey | Filled and sewn by hand in New Jersey | Filled and sewn by hand in New Jersey. | Filled and sewn by hand in New Jersey | Filled and sewn by hand in New Jersey | Filled and sewn by hand in New Jersey. |
| Trial | 60 nights to decide | 60 nights to decide | 60 nights to decide. | 60 nights to decide | 60 nights to decide | 60 nights to decide. |
Frequently Asked Questions
What is a circadian rhythm in simple terms?
A circadian rhythm is a change in your body that repeats about every 24 hours. Sleep and waking run on one, and so do your core temperature, your melatonin and cortisol, your appetite and your digestion. A clock in your brain keeps the time, clocks in your organs follow it, and daylight resets them every morning.
What is the difference between a circadian rhythm and a sleep cycle?
They work at different scales. A circadian rhythm covers a whole day and sets when you get sleepy and when you wake. A sleep cycle happens inside one night, running about 90 minutes through light sleep, deep sleep and dreaming sleep, four or five times before morning. The rhythm decides when the night starts, and the cycles fill it.
How long is a circadian cycle?
About 24 hours, and the word about is doing real work. Czeisler and colleagues measured the average human period at 24.18 hours in Science in 1999, in people living under controlled lighting. That is eleven minutes of drift a day, or more than an hour a week. Daylight corrects it, which is why morning light matters more than any supplement.
What controls your circadian rhythm?
Light, more than anything else. It reaches a master clock of about 20,000 nerve cells in your brain, which sets melatonin production by how much light your eyes take in. Morning light pulls the clock earlier and evening light pushes it later. Meals, exercise and temperature nudge it too, and they work best when they agree with the daylight.
What happens if your circadian rhythm is off?
You rarely feel out of rhythm. You feel wide awake hours after you meant to be asleep, flattened by mid-morning, hungry at odd times and foggy through the afternoon. Jet lag, shift work and a weekday alarm all produce it. If your sleep timing has been wrong for months, take it to a clinician, because circadian rhythm sleep-wake disorders are a real clinical category.
Can a pillow or a comforter change your circadian rhythm?
No. A pillow, a comforter and a mattress all leave your body clock where it was, and any product sold as a way to reset one is selling. What bedding can do is smaller and still worth having. Your core temperature falls at night as part of the rhythm, and the heat leaves through your skin. Bedding that holds that heat in works against the fall.
Your temperature falls at night whether the bed helps or not. An organic wool pillow lets the heat off your face instead of holding it there, in a cover you can wash.
Shop the Organic Wool Pillow