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Circadian rhythm disorders are relatively common among individuals of all age groups, professions, and health conditions across the US. Over 70 million adults in the US now suffer from one kind of circadian rhythm disorder or another. It affects almost all biological processes through disruption of sleep-wake cycles in individuals.

People with any circadian rhythm disorder suffer from lack of sleep, too much sleep or simply not enough sleep at the right time. Circadian rhythm disorders often cause bodies to only respond to their internal biological clocks. Unlike insomnia, the affected individuals experience a normal quality of sleep when they listen to their inner clocks.

What is a Circadian Rhythm Disorder?

Research shows that several sleeping disorders trouble people all across the globe. The kind of circadian rhythm disorder that might affect you does not depend on your age or your geographic location. It depends mainly on your clock genes, several hormones, and other intrinsic factors. Investigations on sleep disorders for over the last decade or more show that a complex network of neurobiological factors is responsible for the genesis of circadian rhythms in people and their subsequent disorders.

One could say that the circadian rhythms are the natural behavior pattern changes, mental and physical changes that a person goes through every day. The primary stimulus for this change is external light. The most common of all circadian rhythms is staying awake in the mornings and sleeping at night. If you think that only human beings experience this rhythm, you are wrong.

Every animal, plants and other tiny microbes experience circadian rhythms that depend on external light cues. The highly conserved structure of biological clocks among the animal and plant life are responsible for the regulation of these biological clocks. Although biological clocks and circadian rhythms are not synonymous, they are closely related to one another.

What are the Simplest Classifications of Circadian Rhythm in Animals?

You must have already seen the simplest breakdown of the 24-hour cycles depending on the maximum activity peak and sleep onset time of different species.

  • Diurnal. Human beings tend to be grossly diurnal. We are active during the daytime, irrespective of the delays or the advances in our circadian clocks.
  • Nocturnal. These are the animals that are active during the night. Nocturnal animals include hyena, fox, and possums.
  • Crepuscular. Cats and some big cats (this explains why your cat always wants you to wake up and play with them at 4 am!) are crepuscular in nature. They are active during dawn and dusk when their target preys are more likely to forage or return home from foraging.

The peak activity hours and resting hours of any species depend on their habits as functions of evolution and adaptation. While circadian rhythms depend on endogenous regulations and exogenous signals, the biological clocks may as well depend upon multi-trophic (multiple feeding) systems.

Gene oscillations, lunar rhythms, tidal rhythms, and ultradian rhythms are also responsible for the control and modulation of the circadian clock. The inclusion of so many control factors opens up the possibility of a few things that may go wrong with the circadian rhythm of a complex organism, like a human being.

How to Distinguish Circadian Rhythm from Regular Repetitive Traits?

You might wonder how it is possible for a body to know when it is time to wake up, eat and sleep. It is possible due to the presence of a master clock in the brain of the animal. The case is different for plants since they do not have a defined neuron system or a central nervous system (CNS). In case of animals, the autonomous circadian activity at the cellular level can affect core temperature, hormone production, cell repair and brain wave activity.

The circadian rhythm is a result of several clock genes that remain unchanged throughout several species and genus. However, you must also understand that not all recurring behavior or changes are products of the circadian rhythm. To be a part of the circadian rhythm, your biological rhythm must fulfill the following requirements;

I. The biological rhythm has to be internal, self-dependent and close to 24 hours in duration.

The rhythm should withstand constant conditions with the approximate period of 24 hours. This period of free-running is tau, and it should persist in the absence of external cues. Generally, the Taus are longer than 24-hour periods for diurnal creatures and nocturnal animals, and the free-running period is usually shorter than 24 hours.  

II. The circadian rhythms should be entrainable.

Although the circadian rhythms are endogenous, they are not entirely recalcitrant to training. They should be entrainable by external cues. The simplest of all exemplification of this trait is when subject travels through several time zones. The person usually suffers from jetlag, and then they can train their internal circadian clocks to respond to local time

III. These rhythms should follow specific physiological temperature changes.

The biochemical reactions at the cellular level are highly sensitive to external temperatures. The temperature will affect the levels of kinetic energy at the molecular levels. Now, irrespective of these changes, the molecular clocks of most mammals can maintain a 24-hour periodicity. The increase or decrease in the rate of a reaction in the biological system in response to a variation of 10-degree centigrade is Q10 temperature coefficient. It is a measure of the compensating effect of the circadian rhythms.

What Are The Two Categories of Circadian Rhythm Sleep Disorders?

The Circadian Rhythm Sleep has two main categories, namely;

Extrinsic Type Circadian Rhythm Disorder

While a deviation of the natural system from these circadian rhythms almost always leads to devastating physiological and mental changes, not all the causes of this deviation are intrinsic. Sometimes, some external cues cause circadian rhythms to deviate from the norm.

Shift Work Sleep Disorder

This is a typical example of an extrinsic type of circadian rhythm disorder. It is highly circumstantial. It mostly affects people working in night shifts or changing shifts. Doctors, nurses, call center workers, customer service agents, police officers, firefighters, and emergency responders often show signs of heightened fatigue due to different shift work sleep disorder.

The Intrinsic Type Circadian Sleep-wake Rhythm Disorder

A majority of the sleeping disorders have an internal or built-in cause.

Advanced sleeping phase disorders (ASPS). This is when people easily fall asleep during the early evenings and have wake-up times in the early hours of night or dawn. People with ASPS often fall asleep by 9 pm and wake up by 2 am or 3 am.

Delayed sleeping phase syndrome (DSPS). DSPS causes people to stay up till late. Adolescents and young adults often exhibit early symptoms of DSPS. They show later than usual sleep onset time and wake up much later too. They often experience peak alertness in the middle of the night.

Non-24-hour sleep-wake disorder. People, suffering from hypernychthemeral syndrome or a non-24-hour sleep-wake disorder can cause them to fall asleep much later each day. The sleep onset and offset times usually shift by an hour or more each day.

Irregular sleep-wake rhythm. While the total sleep time for a person tends to be regular or typical for the person’s age, people with irregular sleep-wake rhythms usually fall asleep more than once per day.

There are several types of circadian rhythms among people even though the hours can be very diverse. There are morning people, who like to wake up early and go to bed early. Then there are the evening people, who prefer to stay up until late and wake up late. Nonetheless, people with regular circadian rhythms find it easier to wake up at a fixed time each morning. They also find adjusting to new routines or a new time zone much easier than people with circadian rhythm disorders. We will learn more about the different types of CRDs in the next section.

What are the Different Circadian Rhythm Abnormalities?

Since there are various types of natural biological clocks, there are several types of circadian rhythm phase disorders that can affect a group of people.

People often find it difficult to understand what kind of a disorder they might be suffering from without the help of an expert. If you feel you are a victim of one such disorder, seek the help of a professional.

Delayed Sleep Phase Disorder 

Patients with delayed sleep phase disorder or syndrome (DSPD or DSPS), experience a standard quality of sleep when they are allowed to follow their biological clocks. Usually, they experience a delay of about 3 to 6 hours in sleep onset time. Understandably, their wake-up times also experience a delay to compensate for delayed sleep onset time. People with DSPS are not lazy or unmotivated. Their day starts at least 3 hours later than it does for the regular people.

They conform to a 24-hour schedule, but they are unable to fall asleep or wake up at socially acceptable hours. 10% of the people with chronic insomnia suffer from DSPS. Their sleep-wake cycle delays are independent of their age, sex, geographic location and profession. They find it impossible to sleep even when they are physically tired at the end of a day.

People with DSPS are likely to fall asleep and wake up at pre-set “usual” time as per their internal body clocks. They find it impossible to retrain their sleep-wake cycles. They often get lesser than necessary sleep per day, since they tend to wake up at fixed times irrespective of the amount or duration of sleep and as a result often complaint about day-time sleepiness.

Over 15% of the teens in the US currently suffer from delayed sleep phase syndrome. A lack of morning light exposure can also lead to DSPS in individuals, so it is a common occurrence in the Scandinavian countries that experience six months of light and six months of darkness. It is also common among those suffering from depression and anxiety disorders.

Advanced Sleep Phase Disorder 

Advanced sleep phase disorder (ASPD) is a classic circadian phase disorder. The patients experience an advancement of their sleep-wake cycle. Their sleep onset time can advance between 9 pm and 7 pm and their offset time can advance between 5 am, and 3 am.

Some people wake up as early as 2 am after completion of their five REM and five non-REM sleep cycles. Just like in case of DSPD, people with ASPD have no troubles with the quality of sleep. They fail to stay up after a point in time, and this often causes social isolation, family problems and other issues due to evening sleepiness.

A study shows that when researchers deprived a group of patients from getting early sleep, they still woke up at their regular hour in spite of not completing their sleep. Environmental cues do not impact individuals with ASPD as much as their genetic makeup. There is a strong genetic link with early sleep onset that these people can pass on to their next generation 50% of the times.

While some erroneously believe that people suffering from ASPD are merely early larks, you can get your polysomnography test from a reputed sleep clinic to find out if you have a circadian sleep disorder. This test notes the limb movements, breathing patterns, heart rate and brain activity of the patient to confirm whether the subject has ASPD or any other sleeping disorder. It is a standard test that can also unravel tendencies of sleep apnea and insomnia in the subjects.

Psychologists, psychiatrists, and ordinary people often correlate waking up unusually early with the precedence of depression. However, investigations characterize ASPS or ASPD as a genetic condition that might lead to depression due to increasing social alienation and lack of social interactions.

Non-24-hour Sleep-Wake Disorder 

The non-24-hour sleep-wake disorder or non-24 is a chronic non-24-hour biological rhythm pattern that qualifies as a circadian rhythm disorder. It affects the regular synchronization of the internal biological clock with the external stimuli. One of the most common types of the non- 24-hour sleep-wake disorder is the free running type or hypernychthemeral syndrome.

People with non-24 usually have body clocks that have a slightly longer than the usual 24-hour night-day cycle. It has steady graduation. For example – people with a 25-hour body clock will sleep one hour later the first day, 2 hours later on the second day and this will continue up until they complete their 24-hour cycle on the 24th day. It is a chronic and steady delay of their sleep onset time due to a slightly longer circadian clock.

People with this affliction often find it difficult to fall asleep at the correct time. About once or twice a month the person can experience a period of great sleep since their sleep-wake cycle realigns with the external 24-hour photoperiod. Nonetheless, this is a temporary relief since their cycles soon start shifting by an hour once they achieve a perfectly aligned sleep cycle. Since they have no way to entrain their circadian clocks with the help of external cues, their natural circadian rhythms are often off by a couple of minutes or even hours.

Sleep experts recommend biochemical circadian rhythm signal tests to ascertain the presence of non-24 hour among people. The levels of melatonin in the body over a period can help the experts understand whether the person has non- 24-hour sleep-wake disorder or some other circadian rhythm disorder.  

Irregular Sleep-Wake Rhythms 

Although rare, an irregular sleep-wake rhythm is a form of circadian rhythm disorder in human beings. Many still confuse this with polyphasic sleep patterns. The main difference between irregular sleep-wake rhythm and polyphasic sleep patterns is the lack of special hours of sleep during a 24-hour day.

There is no particular sleep-wake cycle or prolonged sleep phase at night. There is a sheer absence of regularity of sleep patterns. You can easily find yourself wondering if you suffer from irregular sleep-wake rhythm after a long flight to another part of the country or world. When broken sleep or disruptions in sleep follows a long trip, it is usually jet lag and not irregular sleep-wake rhythm.

In fact, the latter is quite rare and severe. Experts often refer to it as the invisible disability. ISWD has several causes that further weaken a weak circadian clock. It is essential for someone with ISWD to consult with a doctor and a sleep expert since they usually suffer from other neurobiological diseases or neurodegenerative diseases like Alzheimer’s, dementia, mental retardations and brain damage.

The risk of ISWD increases with advancing age, mainly due to the sharp increase of co-morbid health conditions. The confirmed diagnosis usually needs polysomnography and actigraphy. The lack of proper quality sleep is one of the defining characters of this disorder, which makes it unique among other circadian sleep disorders. Restorative sleep with irregular sleep-wake rhythm is almost impossible without medication and prolonged treatment.

Shift Work Sleep Disorder 

Shift work sleep disorder (SWSD) used to be a defining trait of people working on continuous rotating shifts. It is a classic circadian rhythm disorder that consists of insomnia and work-time sleepiness. In a majority of the cases, the rotating work hours often coincide with the typical sleep period of the individual. Since the working shifts and their rotations are highly unpredictable, the defining characters of SWSD (barring a few) are highly variable.

There is a certain degree of sleep deprivation that always occurs with shift work sleep disorders. In many cases, nightshift workers are unable to get good quality sleep in the morning-noon due to familial and social responsibilities. In other cases, the internal biological clocks prevent these workers from getting restorative sleep. According to an International Agency for Research on Cancer study in 2007, shift-work increases the risks of cancer in people of almost all ages. The chances increase with increasing age.

Over the years, doctors, sleep researchers, and scientists have associated several medical co-morbidities with SWSD. Increasing sleepiness during work hours, slow response to external stimuli, non-natural hunger-satiation response, lack of restorative sleep and several other circadian misalignment symptoms often characterize the occurrence of shift-work disorders in people.  

What are the Leading Causes of Circadian Sleep rhythm Disorders?

Several times people mistake the cause of circadian rhythm disorders to be external. There are several doctors across the globe, who believes that disruption of the circadian rhythm in an individual is a result of the lifestyle problems or behavior problems only.

Sadly, the causes are not so simple and just taking away the external stimuli does not restore the intrinsic biological rhythm in people. There are myriads of causes to the simplest of the simple circadian rhythm disorders.

1. Sensitivity to zeitgebers. Zeitgebers directly translates to time givers in English. These include external time cues like light, temperature, and darkness. Disruption of sensitivity towards these time givers is one way to explain the most common circadian rhythm disorders in human beings. Several studies on blind subjects and their intrinsic circadian rhythms show why it might be challenging to entrain the internal biological clock to light cues in the absence of a complete visual system.

2. Environmental factors. This is the next most common cause of some sleep-wake disorders in human beings. High room temperature bright light and more noise are not conducive towards better sleep. These are important factors that affect the quality of sleep in night workers and shift workers.

3. Imperfect pacemaker function. Biological pacemakers maintain the balance of the clock gene products depending on the time. They are necessary for the internal coupling mechanisms of the body. Your sleep-wake cycle is inherently dependent on the temperature cycle and light cycle. The lack of a pacemaker function can potentially compromise the balance. 

4. Frequent travelling. Short distance trips, from one region of the state to another might not be as disruptive for an individual. However, moving from one country to another, crossing several time zones can be challenging to your internal body clock that is already set for a 24-hour period. When the night becomes day, your body often feels confused as the biological clock says its night, yet the external light cues state it’s day. It is our old friend, jetlag. The intensity of the effect depends on how far you are moving away from your time zone.

5. Neurobiological factors. We cannot overlook the possibility of a solely neurological or a neurobiological factor for the disruption of one’s circadian rhythms. Alzheimer’s is one of the few diseases that can always contribute to sleep-wake disorders. Researchers have often cited several neurobiological causes for sleep disorders like Parkinson’s, Huntington’s and sclerosis.

Lifestyle and social causes are of course the contributing factors, but they cannot act without the help of internal mechanisms to disrupt the sleep-wake cycle in people. Blue-light devices, high-intensity CFLs and LEDs, traffic noise and improper eating habits significantly disrupt circadian rhythms in adults, adolescents and children alike. More and more children are staying up late and going to school tired.

Mobile phones and tablets are the most significant reason for this. Children are spending more time with these blue light emitting devices. They are suppressing the secretion of melatonin and thus delaying sleep onset on a regular basis. As we have said before, it is possible to entrain your internal biological clock. Therefore, with prolonged exposure to bright blue light up until a certain period can entrain individuals to stay up till late and wake up late, by disrupting their natural biological clock.

Is there any Treatment for the Myriads of Circadian Sleep-Wake Rhythm Disorders?

There are several methods and combinations of techniques that doctors and therapists use to help people to overcome their sleeping disorders. Here are a few of those that might help you too –

  • Behavior therapy. This is a broad term, experts often use to describe several steps of clinical psychotherapy to modulate circadian rhythms in people. Behavior analysts first assess the environmental stimuli, the behavior issues and the internal factors that can influence a person’s sleep-wake pattern. Then it is the responsibility of the behaviorists or the psychotherapists to apply their analyses in clinical therapy which often helps in entraining the internal biological clocks of individual subjects. Assessment of phobias, desensitization technique for anxiety and anger, psychotherapy for depression and other psychological disorders are often parts of the behavior therapy. In case of sleep disorder related behavior therapy, therapists often ask the subject to avoid naps and stimulants throughout the day.
  • Bright light therapy. The lack of bright light in the mornings can prevent the reduction in the levels of melatonin which in turn can cause increased sleepiness throughout the day. On the other hand, exposure to bright lights in the evening or night can prevent proper sleep-wake cycles in the subject. Investigations highlight the importance of blue-light stimulus to increase the synthesis of melatonin in people. Scientists have carefully outlined several processes that can use the blue light and bright light therapy to manipulate the levels of melatonin in the human system. People with delayed sleep phase disorders benefit from exposure to bright light in the mornings and dark treatment in the evenings. Those suffering from ASPD often find it useful to opt for bright light treatment in the evenings to prevent early sleep onset.
  • Dark therapy. This is complementary to bright light therapy. Under dark treatment, people have to use blue-light blocking goggles and surround themselves with non-blue LEDs. It blocks blue and green wavelengths from reaching the eyes and prevents untimely suppression/secretion of melatonin. Blocking blue-light wavelengths in the evening avert melatonin suppression and aids early sleep hours. It is especially helpful for people suffering from DSPD.
  • Chronotherapy. Chronotherapy involves gradual but steady training of the internal biological clock to obey external cues. It means a continuous delay of bedtime by about 10 minutes to 30 minutes in an attempt to correct the sleep-wake cycle. People with DSPS should always try chronotherapy before moving onto hormone therapies and sleep medication. One of the most popular forms of sleep chronotherapy is the controlled sleep deprivation with phase advance or SDPA. According to this rather stringent therapy, people stay up for 24 hours and then go to bed 90 minutes in advance. This process continues till the person achieves the desired bedtime.
  • Medicines. Melatonin, modafinil, and melatonin agonist compounds are particularly beneficial in the treatment of sleep disorders. Increasing levels of melatonin (a natural hormone) correspond with sleep onset. Medications are necessary for adults and sometimes, adolescents, who suffer from chronic circadian rhythm disorders. Taking short-term sleep aids in the combination of wake-promoting agents can help people achieve the necessary sleep-wake times.  

The effectiveness of any therapy depends on the person’s ability to entrain their biological clock, the persistence of comorbid conditions and the efficiency of their therapists. Just like any treatment for other medical conditions, different people may report different levels of benefits from the same therapy or medication. In short, not everyone will benefit equally from these treatments.  

Circadian rhythms are a product of evolution and adaptation for millions of years. The science of sleep includes the study of anatomy, comparative anatomy, molecular biology, genetics, and behavior. Epigenetics, evolution, and environment also play an essential role in chronobiology or the biology of time. The first instance of circadian rhythm as a function of inheritable factors is present in the works of Linnaeus in 1751. He designed flower clocks using flowering plants of selected species. In 2018, researchers and medical professionals are finally able to control the sleep-wake cycles on their human patients by utilizing genetic therapy, light therapy, and chronotherapy in coordination with each other.   

Circadian rhythm is a product of chronobiology in almost all higher creatures. Plants, invertebrates, vertebrates, and even some microbes’ exhibit functions of their internal biological clock. While human beings depend on their circadian clock for their sleep and wake times, plants depend on them for the opening of their stomata, oxygen synthesis, CO2 release, and glucose synthesis. Interestingly, the genes responsible for the organism’s circadian rhythm remain conserved throughout the different stages of evolution.