What is Your Circadian Rhythm and Why is it Crucial for Sleep?

The circadian rhythm refers to our biological clock that drives our sleep-wake cycle.  No matter where we are on the planet, we are biologically programmed to have a sleep-wake cycle that runs almost parallel to the 24-hours that it takes for Earth to complete one full rotation. 

All living things have a circadian rhythm. It helps organisms to detect seasons and tell time even when external cues are absent.

Most of our organ systems even function based on circadian rhythms, especially the regulation of temperature, appetite, periods of alertness, and hormone levels.

Let’s talk sleep-wake cycles 

The primary function of the circadian rhythm is to promote wakefulness during the daytime and sleepiness at night. This forms a daily pattern known as a sleep-wake cycle. In healthy humans, this is normally seven to nine hours of sleep followed by 15-17 hours of wakefulness. 

If not for the circadian rhythm, our propensity to sleep would increase throughout the wake cycle, causing us to sleep periodically during the day. Thus, the circadian rhythm promotes wakefulness during the daylight hours while our daily physical and mental activity during wakefulness promotes sleepiness.  

This daily pattern is formed by both internal bodily processes and environmental factors. 

The circadian clock consists of a system of naturally occurring, interacting chemicals produced by the body, with levels that repeatedly fluctuate every 24 hours and signal a time-sensitive response in the hypothalamus.  That time-sensitive response can be biochemical, metabolic (converting food into energy and other constituents), or behavioral and synchronizes with environmental cues, usually light. 

The cues, called ‘Zeitgebers’, help calibrate and maintain our relationship with the 24 hour cycle

It starts in the brain

The neurological basis of the circadian rhythm is located in the hypothalamus, which is a small section of the brain that secretes hormones and regulates bodily functions such as temperature. 

This small brain region, called the suprachiasmatic nucleus (SCN), is constituted of about 20,000 neurons and sits on top of the same pathway involved with vision, giving it the ability to process information regarded light and dark. As light exposure decreases, this region of the hypothalamus can signal to other regions of the brain such as the pineal gland to release melatonin

Since the concentration of melatonin (as well as alertness-promoting hormones like cortisol) fluctuate in a time-sensitive way depending on the light input from the environment, this mechanism ensures that the body’s internal clock runs on a uniform schedule and that we get alert and sleepy at roughly the same time every day.

Circadian rhythm disturbances

There are a couple of ways to disrupt your circadian rhythm. The two most common ways are jet lag and shift work. But even the hour difference of daylight savings time can disrupt your zeitgebers and produce drowsiness until your circadian rhythm re-adjusts. Also, burning the candle at both ends on the weekends by staying up late and sleeping in late can make the transition back to an earlier weekday waking schedule surprisingly difficult — perhaps adding to what makes Mondays so difficult. 

Shift work

Shift workers are constantly adjusting their biological clock to conform with mainstream social cues. They drop their kids off at school at the same time as everyone else. They have their doctor’s appointments at the same times and often schedule social activities to overlap with those on a regular sleep-wake cycle.

For shift workers, finding the time to fit in 7-9 hours of quality sleep while balancing time for friends, family, and other responsibilities can be challenging. 

On top of that, those hardcoded zeitgebers like noise, light, meal timing, temperature make it extremely difficult to communicate to the body that it’s time to be winding down and getting ready for sleep. This can negatively impact sleep latency, which is the amount of time it takes to fall asleep, as well as the duration of sleep. 

About 20% of American shift workers suffer from Shift Work Sleep Disorder (SWSD), a form of insomnia specifically affecting shift workers.

Those who transition from night to daytime shifts often have the most difficult time adjusting. Those who work the night shift consistently will eventually adjust their biological clock to make daytime sleep more restful. However, of the millions of American shift workers, the risk of chronic cardiovascular and gastrointestinal illness are much higher.

Jet Lag

Jet lag is a short-lived circadian rhythm disturbance brought about by long transmeridian flights that travel across time zones. The symptoms of jet lag include fatigue, decreased concentration, gastrointestinal disturbances, and brain fog.

A disturbed sleep cycle isn’t the only factor contributing to jet lag. High altitude and cabin pressure are other factors that aggravate the symptoms. It tends to last a few days if more than three time zones are crossed. The duration of the flight also contributes to the intensity and duration of the jet lag. 

The ability to sleep while on the flight or emulate the sleep/wake cycle that you will be on at your destination reduces the symptoms of jet lag. Flying east, when you lose time, is harder than flying west when you gain time. 

Chronic Circadian Rhythm Disorders

Chronic circadian rhythm disorders include delayed sleep-phase syndrome (DSPS), advanced sleep-phase syndrome (ASPS), irregular sleep-wake cycles, and free-running disorder. Some of these types of circadian rhythm disturbances are genetically based while others are related to age or underlying medical conditions.

The majority of CCRDs are associated with medical conditions such as stroke, mania, depression, intracranial infection, head injury, and the use of central nervous system stimulants or depressants. ASPD is more common in the elderly, FRD is common in blind people, while ISWR is found mainly in intellectually-disabled people and people with dementia. 

There are six distinct circadian rhythm conditions. Besides the transient ones associated with jet lag and shift work, the chronic ones include:

Delayed sleep-phase syndrome (DSPS)

DSPS is defined as the persistent inability to fall asleep and awaken at socially accepted times for periods over six months. These patients can maintain their sleep, once asleep, and have normal total sleep times. This condition is most common in adolescents, teenagers, and young adults.

Advanced sleep-phase syndrome (ASPS)

ASPS is characterized by persistent, early sleep onset (between 6:00 pm and 9:00 pm), with an early morning awakenings generally between 3:00 am and 5:00 am. ASPS is less common than DSPS and is found more often in elderly patients and in those suffering from depression.  

Irregular sleep-wake schedule

An irregular sleep-wake schedule demonstrates multiple sleep episodes without evidence of cycles. Sleep, wakefulness, and meals are irregular. Total sleep time is normal in all cases.

Free-running disorder (FRD)

FRD is known as non-24-hour sleep-wake syndrome and shows a chronic, progressive pattern of one to two-hour delays in sleep onset and wake times. 

Of the four chronic circadian rhythm disorders (CRSDs), DSPD is the most common, accounting for 83% of the chronic circadian disorders, followed by ISWR (12%). ASPD and FRD are rare and diagnosed in fewer than 2% of CRSD patients, respectively. 

Some reports suggest that circadian disturbances can lead to cancerAlzheimer’s disease and mild cognitive impairment.

What causes circadian rhythm disturbances?

  • Altered or disrupted sensitivity to environmental cues.
  • Disruption of other bodily functions.
  • Sleep environment such as too much light, noise, room temperature is too high. Night workers often experience this.
  • Travel. Crossing multiple time zones or traveling in an eastward direction. The rate of time adjustment is 1.5 hours per day going east and 1 hour going west. Other jetlag factors include the time of day at the destination; ability to sleep while traveling; and light exposure.
  • Neurological diseases such as Alzheimer’s disease are associated with circadian rhythm disturbances. The phenomenon of sundowning occurs when there are frequent awakenings.
  • Circadian rhythm disturbances are common in children with cancer. Such disturbances are also seen after an injury to the brain from trauma or radiation.
  • Genes play a role in some familial forms of circadian rhythm disorders.

What are some of the signs and symptoms of circadian rhythm disorders?

Total sleep time and duration of dysfunction

In shift workers, sleep time tends to be less than advanced sleep phase syndrome (ASPS) and delayed sleep-phase syndrome (DSPS). Remember, the duration of sleep is the same for ASPS and DSPS, but the onset of sleep is different for these two. A condition that lasts greater than six months is considered chronic.

Peak alertness at irregular hours

Peak alertness is in the evening with DSPS, in the morning with ASPS, and unpredictable for irregular sleep-wake cycles. 

Daytime sleepiness

Daytime sleepiness is found in all circadian rhythm disorders. Concentration may be poor, which impairs performance. Headaches and poor coordination can occur, as well.

Anxiety or depression can lead to circadian rhythm disorders

Depression or anxiety can lead to either DSPD or ASPD because excess or inadequate sleeping is one symptom for both conditions. Additionally, the medications used to treat depression or anxiety can affect the biological clock.

Cognition

Cognition or higher thought processes and decision-making can take a downward turn. When people cannot get to sleep but have to awaken at a scheduled time, such as for school or work, then the daytime may be filled with excessive sleepiness because their circadian rhythm was disturbed. This is common in adolescents and adults. Excess daytime sleepiness can impair concentration and poor decision making, leading to higher rates of both work-related and car accidents.

Self-treatment 

Self-treatment through supplements and drugs in most cases does not address the root cause of the sleep problem.

Using alcohol, OTC sleep aids, prescriptions, or allergy drugs to help you sleep can further disturb one’s natural circadian rhythm and lead to a cycle of dependency.  

Medication history

Medication history and timing of ingestion. Drugs for asthma, caffeine, antidepressants, steroids, nicotine, and others can affect your sleep. Studies show that timing of medication ingestion can affect the performance of the drug and might keep you awake when your natural tendency might be to go to sleep.

What tests are used to diagnose circadian rhythm disturbances? 

Sleep tests to diagnose disorders

One of the most basic sleep tests involves keeping a log of one’s sleep that captures the time and duration of sleeplessness, identifies sleep-wake cycles in a normal environment, and describes subjective alertness over two weeks. 

Another test, called the Multiple Sleep Latency Test (MSLT), is used for the objective measurement of sleepiness. 

The Epworth Sleepiness Scale is a simple self-assessment test that measures how likely you are to doze off during particular activities. It’s widely used in sleep science.

Imaging studies

Brain imaging such as MRI or CT are used to scan and evaluate for suspected neurological diseases. Neurological diseases such as Alzeimer’s and Parkinson’s disease can be diagnosed with these tests. Both can cause circadian rhythm problems or lead to them.

Actigraphy

Actigraphy is a sleep evaluation technique used for tracking and measuring your sleep-wake cycles over a period of time. It uses a small device attached to the wrist and measures activity and movement throughout the day, typically with an accelerometer.

Overnight polysomnography

This set of tests is used for identification of obstructive sleep apnea (OSA), narcolepsy, periodic limb movements of sleep (restless legs), or sleep-state misperception all of which affect circadian rhythms. It involves using multiple sensors, including EEG to track and measure brain waves, pulse, blood pressure, breathing, and leg and eye movements. This is considered the gold standard for understanding an individual’s personal sleep patterns.

How to fix my circadian rhythm?

Treatment for circadian rhythm disturbances is primarily behavioral. DSPS usually resolves in adulthood. ASPS is managed with a combination of behavioral therapies and medications. 

For a circadian rhythm problem like jet lag, in most cases, time and good sleep hygiene are all that’s needed. For Shift Work Sleep Disorder, often, coming up with a plan for how to create a sleeping environment that allows you to get the necessary seven to nine hours of sleep you need each night is the first step. 

For underlying medical conditions, it’s best to speak to your doctor or healthcare provider about your sleep troubles.

Behavioral therapies for Circadian Rhythm Disorders

Behavioral therapies can be some of the most effective ways to recover a desired circadian rhythm. Some of these include

  • Cognitive-behavioral therapy.
  • Bright light therapy.
  • Chronotherapy, the process of determining the optimal time to go to bed or receive chemotherapy based on circadian rhythms. Pushing bedtime back or moving it up gradually may improve sleep disorders from circadian disturbances.
  • Enhancing environmental cues.
  • Lifestyle changes such as avoiding caffeine and alcohol as well as increased exercise during the daytime.

Glossary

Cerebral cortex: Is part of the brain known as grey matter. It has two sections or hemispheres that are connected. It plays a key role in attention, perception, awareness, thought, memory, language, and consciousness.

Hypothalamus: A region of the brain that coordinates both the nervous system and the activity of the pituitary- another small structure in the brain that secretes hormones. It controls body temperature, thirst, hunger, and is involved in sleep and emotional behavior.

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