What is a Circadian Rhythm?
Understand how your body clock functions and controls your sleep cycles.
Mar 19th, 2021 •
Expert Insights from Dr. Anna Pickering a medical writer who received an honors baccalaureate of science in biochemistry and biophysics, minoring in chemistry, from Oregon State University and her doctorate in cell and molecular biology from the University of Hawaii at Manoa’s John A. Burns School of Medicine.
Our ability to function in a happy, healthy manner each day has a lot to do with our body’s natural clock. This internal clock is responsible for triggering hunger and sleepiness. It’s also linked with overall short- and long-term health.
When the biological clock is disrupted, there’s a direct negative impact on body functions that disrupt sleeping and eating patterns. Not only will we feel tired and unfocused when our body clock is off, we will also have an increased risk for chronic health problems, bipolar disorder, depression, obesity, diabetes, and sleep disorders.
This natural clock is referred to as the circadian rhythm.
Virtually all animals and plants have a built-in circadian rhythm. It regulates when the body wants to sleep and wake based on external cues, known as Zeitgebers (a German word meaning “time-givers”). The most important cue for sleep is daylight.
Related: Sleep wake homeostasis
The brain’s internal circadian clock (also known as the biological clock, body clock, circadian pacemaker, circadian system, circadian oscillator, etc) is centred in the hypothalamus region of the basal forebrain. It uses these Zeitgebers to naturally synchronize or reset itself each day to within just a few minutes of the Earth’s 24-hour rotation cycle. The word “circadian” comes from the Latin words meaning “about a day.”
About 25% of people have a circadian period which is slightly less than the 24-hour day, and 75% have a circadian period slightly more than 24 hours. Early studies showing longer, closer-to-25-hour cycles did not account for artificial light and more recent research shows cycles closer to 24 hours, but actual numbers vary by study.
“Your brain’s circadian clock is responsible for quite a bit,” says Dr. Pickering. “It helps to regulate sleeping and feeding patterns and dictates three important growth hormones related to your sleep.”
These hormones affected by the circadian clock relating to sleep include:
- Melatonin – produced in the pineal gland in the brain, chemically causes drowsiness and lowers body temperature)
- Cortisol – produced in the adrenal gland, used to form glucose or blood sugar and enable anti-stress and anti-inflammatory functions in the body
- Growth hormone – essential to the repair and restoration processes of the body, usually secreted during deep non-REM sleep, along with other hormones like testosterone.
The brain’s circadian clock also regulates alertness, core body temperature, brain wave activity, hormone production, regulation of glucose and insulin levels, urine production, cell regeneration, and many other biological activities.
When Are We Meant to Sleep?
Humans are naturally active during the daytime and our circadian rhythms reflect this. Generally speaking, for sleep to occur in the “right” part of the circadian cycle, the time of minimum core body temperature and maximum melatonin concentration should occur towards the end of the sleep period.
As a rough guide, core temperature usually reaches its minimum around 4:30-5:00 am in human adults, and melatonin typically begins to be produced around 8:00-9:00 pm, stopping around 7:00-8:00 am (see diagram below). The deepest tendency to feel sleepy occurs in the middle of the night, around 2:00-3:00 am, along with a shorter and shallower period of sleepiness (often referred to as the “post-lunch dip”) about twelve hours later, around 2:00-3:00 pm in the afternoon.
A circadian rhythm will stay balanced so long as we go to bed in the evening and rise in the morning around the same time each day—getting 7-9 hours of sleep per night.
“Sunlight is crucial to your body’s circadian clock; it works as an external clue that helps your body regulate your circadian rhythm,” says Dr. Pickering.
Circadian rhythms are controlled by a part of the brain that responds to lightness and darkness. Physically, the circadian clock is located in the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain, one in each brain hemisphere. The SCN is a tiny pinhead-sized area, containing just 20,000 or so very small neurons, but it has the responsibility for sending signals to several other parts of the brain.
Circadian clock genes are also located in the liver, kidneys, pancreas, muscles, etc., however, it’s the SCN that instructs the rest of the body to stay on schedule and how to take cues from the environment. This allows it to regulate the daily sleep-wake cycle, body temperature, hormone production and other functions.
The individual neurons that make up the SCN have been found to exhibit a near-24-hour rhythm of activity, suggesting that the clock mechanism actually works on a sub-cellular level. When dissociated from the SCN, the individual cells follow their own intrinsic 24-hour rhythms, but, when incorporated into the SCN, they all fire in synchrony. In experiments on mice where the SCN is completely removed, the mice (which are normally much more active during the nighttime and sleep more during the day) show little or no preference for their active time and sleep time, and their activity is sporadic and apparently random throughout the day and night.
Other functions of the SCN
As well as regulating hormone production, body temperature, etc, the SCN also sends out an alerting pulse throughout the day (sometimes referred to as the circadian alerting system) which counteracts the increasing homeostatic sleep pressure. These alerting pulses from the SCN reach their peak about 2-3 hours before one’s habitual bedtime (sometimes referred to as the “wake maintenance zone“), which serves to offset the homeostatic drive that has been continually accumulating throughout waking hours, allowing for continued alertness late into the evening.
As the evening progresses, though, the SCN’s alerting pulses start to weaken, melatonin production in the pineal gland increases (also under the direction of the SCN), and the “sleep gate” (also known as the primary sleepiness zone or sleep onset zone) opens, and the urge to sleep increases dramatically.
There are also other secondary or peripheral biological clocks throughout the body, such as in the liver, heart, pancreas, kidneys, lungs, intestines, and even in the skin and lymphocytes, all of which show natural daily oscillations. These organs are largely entrained independently by factors like the timing of meals, ambient temperatures, etc, rather than by the light-dark cycle, but the central coordination and synchronization of these secondary body clocks is still carried out by the suprachiasmatic nuclei.
The main circadian system in the SCN in turn receives multiple feedbacks from these various organs, in a complex system of reciprocal interactions. Chronobiology, the relatively new science of timing medical attention to various organisms of the body depending on the most propitious time of day for those particular organs, has shown very good results in improving the effectiveness of treatments.
The role of sunlight
The circadian clock checks its accuracy each day using external cues, principally the light-dark cycle. Exposure to natural daylight stimulates a nerve pathway from special photoreceptive ganglion cells in the retina of the eye. The SCN is triggered when light enters the eyes, and this information is transformed into neural signals that set the body’s internal temperature. It’s through these fluctuations in body temperature that the circadian rhythm is regulated.
These cells in our eyes contain a unique light-sensitive pigment called melanopsin, and are most sensitive to short wavelength “blue light”. Even many blind people can respond to these light-dark cues, as the photoreceptive cells in their eyes can usually recognize daylight, even through closed eyelids. The light-dark signals are sent via the optic nerve to the suprachiasmatic nucleus, which uses them to reset its own circadian clock each day.
When the sun goes down, the SCN triggers the body to begin releasing melatonin, which helps the body become tired in preparation for sleep. Sunlight, on the other hand, restricts the release of melatonin and prompts feeling awake and energized.
The biological clock does not actually require light to function – the circadian cycle persists quite accurately even when individuals are completely cut off from daylight. The light-dark cycle (in concert with other cues like meals, ambient temperature, etc), merely acts as an external cue to resynchronize or entrain the timing of biological rhythms, and to prevent small timing errors from accumulating. Without this important check, the circadian system can become seriously unbalanced.
For example, the much dimmer illumination of artificial lights is not usually sufficient to trigger this reset of the circadian clock, which is why night shift workers never really fully adapt to their unnatural sleep patterns (see the section on Shift Work). It has been shown that simply increasing day-time lighting intensity in workplaces and care homes for the elderly can significantly improve their sleep regimes, reduce cognitive decline and improve mood disorders.
Circadian rhythms can change overtime as you age. They also may be altered by common disruptions such as traveling, working the night shift, or adjusting to Daylight Saving Time.
Changes in Circadian Rhythm as You Age
“Most people will find as they age into their senior years, they have more energy in the morning hours and less energy in the evening,” says Dr. Pickering. “This isn’t true for everyone, but it is expected that your circadian rhythm will change as you age.”
Irregular Sleeping Patterns
When sleeping patterns change, the circadian rhythm will be disrupted. There are various situations in which this is common to occur.
- Traveling: Circadian rhythms can become disrupted from traveling into different time zones—causing jet lag.
- Working the Night Shift: When a person works a night shift he/she may find sleeping during the day to be difficult, and eating patterns may also change. Night shift workers are more susceptible to experiencing sleep disorders (like shift work sleep disorder), mood disorders, weight gain, health issues, and chronic fatigue.
- Daylight Saving Time: DST is an instance during which a person’s circadian rhythm may become disrupted. It can take a while for the body to adjust to getting up and going to bed an hour earlier or later in the day.
- Adjusting to College: College students face disruptions to their circadian rhythms due to their strenuous schedules that oftentimes require studying late into the evening.
- Social Jet Lag: Social jet lag is a common circadian rhythm disruption that occurs when a person decides to stay up later than is typical on the weekends, and/or changes the time that he/she wakes up, exercises, eats, etc.
Having a disrupted circadian rhythm can cause numerous difficulties in a person’s day-to-day life, but there are ways to help improve circadian rhythm functioning.
To improve the functioning of your circadian rhythm and help it stay on a regular schedule, there are a few steps you can take.
1. Determine Your Chronotype
Circadian rhythms may be adjusted by up to two hours or so either way according to an individual’s chronotype. Some people (often known as “larks” or morning people) tend to wake up early and are most alert during the first part of the day. Others (“night owls” or evening people) are most alert in the late evening and prefer to go to bed late.
By some estimates, as many as 20% of people fall into one of these two categories. In these people, the timing of their circadian period is shifted completely (an effect that is at least partly encoded in their genes), so that morning people wake at a later stage in their circadian day, and are therefore much more alert on waking. Evening people, on the other hand, wake too early in their circadian day, and so are less alert and perform poorly in the morning.
Typically, this variation is limited to a couple of hours earlier or later than the average; those with extreme body clocks may have difficulty participating in normal work, school or social activities, and are considered to suffer from circadian rhythm sleep disorder (see the section on Sleep Disorders).
Since each person’s body operates to its own unique rhythm, it’s important to determine your own circadian rhythm before attempting to try to improve its functioning. Chronotypes are also related to genetics. If you can align your sleep pattern to your own circadian rhythm, it will help increase the amount of deep and REM you’re able to have each night.
2. Develop a Consistent Daily Schedule
One of the best ways to keep a circadian rhythm functioning well is by going to bed and waking up at the same time each day.
In addition to establishing a consistent sleep cycle, it’s also important to eat and exercise at the same time each day. It’s best to do anaerobic exercises later in the day. Overall, exercise should happen on a consistent basis for greater circadian rhythm functioning.
When it comes to eating, it’s most beneficial to eat the most calories earlier in the day and to finish eating by 6-7 PM with a lighter calorie meal. This will give the body adequate time to wind down for a good night’s rest. For optimal results, it’s important to stay on schedule into the weekend.
3. Manage Light Exposure
Since exposure to light impacts the circadian rhythm it’s important to limit exposure to light in the evening and to expose oneself to light in the morning.
Blue light from television, computer, and phone screens interfere with the body’s release of melatonin, and therefore have a negative impact on the circadian rhythm. Blue light filtering glasses may be worn to help combat this.
In order to gain more energy in the morning, it’s important to expose the body to light in order to stop the release of melatonin. They can be done by taking a walk in the morning, opening a window, or using light lamp therapy.
4. Invest in a Quality Mattress
Since the circadian rhythm is such an important and delicate system, it’s essential not to let anything stand in the way of a potentially good night’s rest that could disrupt your internal clock. This includes blocking out all interfering noises, or creating noise (if that helps), and investing in a quality mattress and pillows.
Nothing can make getting adequate rest more difficult than a worn out, or uncomfortable mattress. Even if a mattress is new, it may not be ideal for you. This is why it’s important to take a mattress quiz to determine your ideal mattress type and to invest in a quality option that meets your unique sleep preferences.
Having the right mattress could potentially improve not only your health but your quality of living and life expectancy. If you’d like some support in getting a better night’s rest we can help find the right mattress for you. Feel free to have a look at our mattress guides to get started.
Dr. Anna Pickering is a freelance medical writer based in Portland, OR working for The Med Writers. She received an honors baccalaureate of science in biochemistry and biophysics, minoring in chemistry, from Oregon State University and her doctorate in cell and molecular biology from the University of Hawaii at Manoa’s John A. Burns School of Medicine. Before transitioning to writing she completed a year of postdoctoral research at Oregon Health and Science University and worked briefly at the biotech startup Ayumetrix.
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Circadian Rhythm Sleep Disorders
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