Circadian Rhythm | annotated by Peter

Circadian rhythms are 24-hour cycles that are part of the body’s internal clock, running in the background to carry out essential functions and processes. One of the most important and well-known circadian rhythms is the sleep-wake cycle.

Different systems of the body follow circadian rhythms that are synchronized with a master clock in the brain. This master clock is directly influenced by environmental cues, especially light, which is why circadian rhythms are tied to the cycle of day and night.

When properly aligned, a circadian rhythm can promote consistent and restorative sleep. But when this circadian rhythm is thrown off, it can create significant sleeping problems, including insomnia. Research is also revealing that circadian rhythms play an integral role in diverse aspects of physical and mental health.

How Does Circadian Rhythm Work?

Circadian rhythms work by helping to make sure that the body’s processes are optimized at various points during a 24-hour period¹. The term circadian comes from the Latin phrase “circa diem,”² which means “around a day.”

Circadian rhythms exist in all types of organisms. For example, they help flowers open and close at the right time and keep nocturnal animals from leaving their shelter³ during the daytime when they would be exposed to more predators.

In people, circadian rhythms coordinate mental and physical systems⁴ throughout the body. The digestive system produces proteins⁵ to match the typical timing of meals, and the endocrine system regulates hormones to suit normal energy expenditure.

The circadian rhythms throughout the body are connected to a master clock,⁶sometimes referred to as the circadian pacemaker, located in the brain. Specifically, it is found in the suprachiasmatic nucleus (SCN), which is in a part of the brain called the hypothalamus. At different times of the day, clock genes⁷ in the SCN send signals to regulate activity throughout the body.

The SCN is highly sensitive to light, which serves as a critical external cue that influences the signals sent by the SCN to coordinate internal clocks in the body. For this reason, circadian rhythms are closely connected to day and night. While other cues, like exercise, social activity, and temperature, can affect the master clock, light is the most powerful influence on circadian rhythms.

Is a Circadian Rhythm the Same As a Biological Clock?

Biological clocks help regulate the timing of bodily processes, including circadian rhythms. A circadian rhythm is an effect of a biological clock, but not all biological clocks are circadian. For instance, plants adjust to changing seasons using a biological clock with timing that is distinct from a 24-hour cycle.

How Does Circadian Rhythm Affect Sleep?

When people talk about circadian rhythm, it’s most often in the context of sleep. The sleep-wake cycle is one of the most clear and critical examples of the importance of circadian rhythms.

During the day, light exposure causes the master clock to send signals that generate alertness⁸ and help keep us awake and active. As night falls, the master clock initiates the production of melatonin, a hormone that promotes sleep, and then keeps transmitting signals that help us stay asleep through the night.

In this way, our circadian rhythm aligns our sleep and wakefulness with day and night⁹ to create a stable cycle of restorative rest that enables increased daytime activity.

BETTER SLEEP, FOR A BETTER YOU.

What Does Circadian Rhythm Affect Besides Sleep?

While the sleep-wake cycle is one of the most prominent circadian rhythms, these 24-hour internal clocks play a vital role in virtually all systems of the body¹⁰.

Research continues to uncover details about circadian rhythms, but evidence has connected them¹¹ to metabolism and weight through the regulation of blood sugar and cholesterol. Circadian rhythms influence mental health as well, including the risk of psychiatric illnesses like depression and bipolar disorder as well as the potential for neurodegenerative diseases¹² like dementia.

There are indications that circadian rhythms have an important influence on the immune system as well as processes of DNA repair that are involved in preventing cancer¹³. Early-stage research indicates that circadian cycles can influence the effectiveness of anti-cancer drugs¹⁴ and that new medications may be able to harness biological clocks to kill cancer cells.

What Happens When Circadian Rhythm Is Off?

When circadian rhythm is thrown off, it means that the body’s systems don’t function optimally.

A disturbed sleep-wake circadian rhythm can give rise to serious sleeping problems. Without the proper signaling from the body’s internal clock, a person can struggle to fall asleep, wake up during the night, or be unable to sleep as long as they want into the morning. Their total sleep can be reduced, and a disrupted circadian rhythm can also mean shallower, fragmented, and lower-quality sleep.

In addition, studies have identified circadian rhythm disruptions as potential contributors¹⁵ to obstructive sleep apnea (OSA), a sleep disorder marked by repeated lapses in breathing. OSA reduces the body’s oxygen levels and causes numerous sleep interruptions through the night.

As a whole, a misaligned circadian rhythm can negatively affect sleep in many ways, increasing a person’s risk of insomnia and excessive daytime sleepiness. Given the essential role of sleep for productivity and overall health, there are often significant consequences when a person’s circadian rhythm is off.

What Can Disrupt Circadian Rhythm?

Disruptions to circadian rhythm can occur over the short- or long-term. Experts have identified a number of types of circadian rhythm sleep-wake disorders (CRSWD) based on their characteristics and causes.

Jet Lag Disorder: This occurs when a person crosses multiple time zones in a short period of time and gets its name from the fact that it is often experienced by people who take intercontinental flights. Until a person’s circadian rhythm can acclimate to the day-night cycle of their new location, they are likely to suffer sleeping problems and fatigue from jet lag.
Shift Work Disorder: Work obligations can cause major disruptions in a person’s circadian rhythm. Shift work, which requires having to work through the night and sleep during the day, puts a person’s sleep schedule directly at odds with the local daylight hours.
Advanced Sleep Phase Disorder: People with this type of disruption find that they get tired early in the evening and wake up very early in the morning. Even if they want to be up later at night or sleep later in the morning, people with an advanced sleep phase disorder usually cannot do so. This disorder is relatively rare, affecting around 1% of people in middle and older age¹⁶, and occurs more frequently in older adults. In some cases, advanced sleep phase disorder may be related to an inherited genetic cause.
Delayed Sleep Phase Disorder: This type of circadian rhythm disruption is associated with “night owls” who stay up late at night and sleep in late in the morning. It is rare among the general population — affecting just 1 or 2 people out of every 1,000 — but impacts up to 16% of teens. The exact cause is unknown but may be related to genetics, underlying physical conditions, and a person’s behavior.
Non-24 Hour Sleep Wake Disorder: This condition occurs primarily in people who are blind¹⁷ and are not able to receive light-based cues for their circadian rhythm. Their body still follows a 24-hour cycle, but their sleeping hours constantly shift backward by minutes or hours at a time.
Irregular Sleep-Wake Rhythm Disorder: People with this rare disorder have no consistent pattern to their sleep and may have many naps or short sleeping periods throughout a 24-hour day. It is frequently connected to conditions that affect the brain¹⁸, such as dementia or traumatic brain injury, that limit the proper functioning of the master clock in the hypothalamus.

As this list demonstrates, there are diverse causes of circadian rhythm disorders. Some circadian disruptions are related to individual behavior, such as for travel or work, that puts sleep schedules out of whack with normal daylight exposure. Other disorders stem from an underlying issue that causes an inability to receive or process environmental cues that regulate the body’s master clock. In certain situations, genetic causes may be involved, or the cause may remain unknown.

How To Maintain a Healthy Circadian Rhythm

While we don’t have full control over our circadian rhythm, there are healthy sleep tips that can be taken to try to better entrain our 24-hour sleep cycles.

Seek out sun: Exposure to natural light, especially early in the day, helps reinforce the strongest circadian cue.
Follow a consistent sleep schedule: Varying your bedtime or morning wake-up time can hinder your body’s ability to adjust to a stable circadian rhythm.
Get daily exercise: Activity during the day can support your internal clock and help make it easier to fall asleep at night.
Avoid caffeine: Stimulants like caffeine can keep you awake and throw off the natural balance between sleep and wakefulness. Everyone is different, but if you’re having trouble sleeping, you should avoid caffeine after noon.
Limit light before bed: Artificial light exposure at night can interfere with circadian rhythm. Experts advise dimming the lights and putting down electronic devices in the lead-up to bedtime and keeping electronics out of the bedroom and away from your mattress.
Keep naps short and early in the afternoon: Late and long naps can push back your bedtime and throw your sleep schedule off-kilter.

These steps to improve sleep hygiene can be an important part of supporting a healthy circadian rhythm, but other steps may be necessary depending on the situation. If you have persistent or severe sleeping problems, daytime drowsiness, and/or a problematic sleep schedule, it’s important to talk with a doctor who can best diagnose the cause and offer the most appropriate treatment.