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Circadian, Infradian and Ultradian Rhythms

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Circadian, Infradian and Ultradian Rhythms

You may have noticed that your body tends to do things in cycles, known as biological rhythms. In women, for example, a menstrual cycle occurs once a month. The body can track the passage of time and change its internal state depending on the elapsed time. It is as if the body has an internal clock that monitors these processes and adheres to an unconscious, predetermined schedule.

These cycles differ in how much time has elapsed between each completed cycle, which defines the nature of the rhythm.

Biological rhythms are natural, biological events or functions of living organisms that follow repetitive patterns. Their purpose is to respond to a period of environmental change. These include circadian rhythms, infradian rhythms, and ultradian rhythms.

Circadian, Infradian and Ultradian Rhythms, Biological Rhythms, StudySmarterThe biological rhythm throughout the day and night, freepik.com/bro.vector

Biological rhythms

Biological rhythms are composed of circadian, infradian, and ultradian rhythms. They differ in how long their cycles usually last. Although there are fixed parameters for how long and how often a cycle occurs in a given period, the body does not always adhere perfectly to these cycles for various reasons.

Endogenous pacemakers and exogenous zeitgebers support these rhythms.

Endogenous pacemakers are genetically determined, innate biological structures that use mechanisms in the body to monitor and adapt to biological rhythms. They are internal.

The suprachiasmatic nucleus (SCN) is a good example of an endogenous pacemaker related to circadian rhythms and helps the body maintain a 24-hour sleep-wake rhythm.

Exogenous zeitgebers are external, environmental stimuli that act in concert with your endogenous pacemakers to trigger biological rhythms.

Light is the best example of an exogenous zeitgeber associated with your circadian rhythm.

Although the body's pacemaker works internally, the environment influences it. Therefore, exogenous zeitgebers can affect your sleep-wake rhythm even though the body's pacemaker and your SCN monitor it.

Circadian rhythms

Circadian rhythms are cycles controlled by daylight that comprise a day and a night, or 24 hours. They essentially occur once every 24 hours. The aforementioned internal clock, also known as the master clock, controls circadian rhythms.

Our sleep-wake cycle is an example of a circadian rhythm.

Circadian rhythms examples

As we mentioned earlier, one of the most well-known examples of a circadian rhythm is your sleep-wake cycle. Your body is not just told when to go to sleep and wake up; it's much more complicated than that.

While you are awake, you use more resources to fuel your body with energy, including exercise, walking, eating, and even thinking. The more complex the mental and physical tasks, the more resources you must use to accomplish them. The digestive system also responds differently when you are awake than when asleep. When you sit down to eat a meal, the digestive system prepares for the meal in advance by secreting and producing certain proteins and hormones.

Ghrelin and leptin are hormones involved in food intake and hunger regulation that initiate meal times.

The body enters a resting state during sleep, and several functions slow down, such as breathing rate, heart rate, and metabolic rate. Your body becomes cooler at night as core temperature drops during the deepest stages of sleep.

Circadian, Infradian and Ultradian Rhythms, Circadian Rhythm Master Clock, StudySmarterMan monitoring clock, freepik.com/pchvector

The suprachiasmatic nucleus (SCN), part of the hypothalamus, is the main bodily pacemaker in mammals and is involved in your sleep-wake cycle because of its sensitivity to light. It regulates your pineal gland, which is responsible for producing melatonin (a key hormone that triggers sleep) based on the light that hits the retina of your eye.

When you are exposed to light, melatonin production is reduced, and you are more likely to stay awake. Conversely, when exposed to less light, more melatonin is produced, promoting sleep.Edgar et al. (1993) found that a lesion of the SCN in squirrel monkeys abolished sleep-wake rhythms, sleep stages, brain temperature, and circadian drinking rhythms and significantly increased total sleep time.

Circadian rhythms may be one of the answers to how we evolved as a species. The system developed to help humans adapt to changes in the environment by anticipating temperature and food resources changes.

Evaluating circadian rhythms

The effects of external elements on circadian rhythms are the subject of research.

Siffre (1975) spent about six months in a cave studying the effects of light deprivation on his sleep-wake rhythm.

By staying in a cave, he no longer had access to light, the exogenous zeitgeber important for helping the SCN induce sleep and maintain an internal body clock.

The natural sleep-wake cycle of 24 hours lengthened to 25-30 hours during his time in the cave, demonstrating the importance of light in keeping this system within its specific parameters. While the circadian rhythm is not entirely dependent on the exogenous zeitgeber, it does work with it to fine-tune it.

When he came out of the room, he believed the date was a month earlier than it was. However, since Siffre experimented on himself, validity and reliability are difficult to measure, which is why he has been widely criticised.

Individual differences exist in the study of circadian cycles. Duffy et al. (2001) studied morning people and evening people. They assumed that morning people simply have their circadian clock set an hour or earlier than evening people, which would explain their sleep patterns. They concluded that 'morning people' are more likely to get up early and go to bed early. This cycle consists of about 6 am and 10 pm. In contrast, 'evening people' are more likely to wake up and sleep later, with a cycle of 10 am to 1 am.

In addition, Buhr et al. (2010) suggested that temperature may play a more critical role than light in determining circadian rhythms. They believed temperature sensitivity exists in the SCN and acts as a universal cue that influences the behaviour of autonomous cells in the body. Resetting temperature (also known as heat shocks) is an evolutionary response to synchronise internal circadian rhythms.

Our body temperature fluctuates in a 24-hour circadian rhythm, and small changes can result in strong signals to our biological clock. Buhr et al. (2010) confirms unfavourable factors influence and control circadian rhythms and suggests a holistic research approach, rather than embracing a general idea.

Infradian rhythms

Infradian rhythms are biological rhythms that occur in a cycle that lasts longer than 24 hours. They usually occur less than once per day and can span weeks, months, and even years.

The menstrual cycle is a monthly infradian rhythm that hormones control.

Interestingly, a woman's menstrual cycle can change depending on whether she lives with other women. Although an internal clock controls the menstrual cycle based on one's biological state, Sabbagh and Barnard (1984) found women's cycles became somewhat synchronised when they lived together. Why this is the case, however, is not known (some suggest it is related to pheromones, but that is up for debate).

However, Yang and Schank (2006) examined data from 186 Chinese women living together in dormitories and found they did not synchronise their cycles during the year. They also reviewed previous literature on this phenomenon and found that synchronisation and cycle variability can have different causes, even pure coincidences and convergences that have been misinterpreted as synchronisation.

Ultradian rhythms and productivity

Ultradian rhythms are cycles shorter than 24 hours. Ultradian rhythms have a shorter period and higher frequency than circadian rhythms and occur more than once every 24 hours.

For example, the frequency and amplitude of the sleep phase during the sleep process. During the night, you often go through different stages of sleep, ranging from light sleep to deep sleep to REM sleep.

Ultradian rhythms are important for health, especially in the case of sleep. Disruption of the natural stages of human sleep can have serious consequences for health, the ability to concentrate and general cognitive functions.

Ultradian Rhythms, Sleep Dream, StudySmarterDreaming during sleep, freepik.com/storyset

Ultradian rhythms example

Ultradian rhythms can be objectively seen in our sleep patterns. Younger people need more sleep than older people.

The sleep cycle consists of the following four phases:

  1. Light sleep – NREM (non-rapid eye movement) sleep, which lasts about five minutes and is when you doze off. This is where you start to relax, and an EEG would show alpha waves.

  2. Light sleep – NREM sleep continues, but theta waves begin. This phase usually lasts about 10 to 25 minutes. Body temperature drops, as do heart rate and respiratory rate. Small bursts of brain activity occur, but usually to reduce the likelihood of waking.

  3. Deep sleep – NREM sleep continues, and muscle tone, heart rate, and respiratory rate relax even more. Delta waves occur here, and this stage can be referred to as slow-wave sleep (SWS). The metabolic rate is at its lowest. This phase is critical for recovery, cognitive function, and restoration of body and health.

  4. REM sleep – In this phase, rapid eye movements are observed, and brain activity increases abruptly. REM is rapid and shows similar EEG results to waking. The brain and eyes are active, but the body is resting and dreaming.

A cycle lasts about 90 minutes and comprises the phases mentioned above.

Overall, ultradian rhythms are biological rhythms responsible for increasing our productivity, energy levels, willpower, and ability to make healthy decisions.

Another ultradian rhythm is the appetite or eating behaviour of humans. Most people eat three meals a day, and their appetite rises and falls with food intake.

Circadian, Infradian and Ultradian Rhythms - Key takeaways

  • Biological rhythms are natural processes or functions in living organisms that follow repetitive patterns to respond to a specific period of environmental change; a biological clock controls them.
  • Circadian rhythms are cycles controlled by daylight that span a day and a night or 24 hours. They occur once every 24 hours. An internal clock, also known as the master clock, controls circadian rhythms.
  • Our sleep-wake rhythm is an example of a circadian rhythm. It determines changes in psychological and behavioural patterns, for example, when we get tired.
  • Ultradian rhythms have a shorter period and higher frequency than a circadian rhythm. The ultradian cycle has a shorter period than a day but is longer than an hour. It repeats during a 24-hour circadian day. It is responsible for increasing our productivity, energy levels, willpower, and ability to make healthy choices.
  • Infradian rhythms are biological rhythms that occur in a repeating cycle that lasts longer than 24 hours. Their rhythm patterns may recur weekly, monthly or annually. The female menstrual cycle has a monthly infradian rhythm that hormones control.

Frequently Asked Questions about Circadian, Infradian and Ultradian Rhythms

An ultradian rhythm has a shorter period and higher frequency than a circadian rhythm. The cycle is repeated throughout a 24-hour circadian day. Circadian rhythms occur once every 24 hours, in general. 

Sleep stages, blood circulation, pulse, heart rate, thermoregulation, blinking, micturition, appetite, arousal, etc., are all ultradian rhythms. They are important in maintaining healthy lives.

The female menstrual cycle is an example of an infradian rhythm, as it occurs once every month. 

Infradian rhythms are biological rhythms that operate in a repetitive cycle lasting longer than a 24-hour period. Its rhythm patterns can be recurrent weekly, monthly, or annually.  

An ultradian rhythm is a cycle that occurs more than once every 24 hours. Sleep occurs in stages that cycle throughout the night every 90 minutes. Therefore, they are an example of an ultradian rhythm. 

Final Circadian, Infradian and Ultradian Rhythms Quiz

Question

What are biological rhythms?

Show answer

Answer

Biological rhythms are natural, biological events or functions of living organisms that follow repetitive patterns. Their aim is to respond to a period of environmental changes. They include circadian rhythms, infradian rhythms, and ultradian rhythms.  

Show question

Question

What are endogenous pacemakers?

Show answer

Answer

Endogenous pacemakers are genetically determined, innate biological structures that monitor and adapt to biological rhythms using mechanisms in the body. They are internal.  

Show question

Question

What is the main endogenous pacemaker in mammals?

Show answer

Answer

The suprachiasmatic nucleus (SCN) which is a part of your hypothalamus, is the main endogenous pacemaker in mammals and is involved in your sleep-wake cycle due to its sensitivity to light.  

Show question

Question

How does the suprachiasmatic nucleus affect the sleep-wake cycle?

Show answer

Answer

It regulates your pineal gland which is responsible for melatonin production (a core hormone involved in inducing sleep) based on light hitting the retina of your eye.


When exposed to light, melatonin production is reduced, and you will be more likely to stay awake. Similarly, with less light exposure more melatonin is produced, which induces sleep. 

Show question

Question

What is the circadian rhythm?

Show answer

Answer

Circadian rhythms are cycles regulated by daylight that comprise one day and one night, or 24 hours. They occur once every 24 hours, essentially. This is governed by an internal clock, also known as the master clock.  

Show question

Question

What is an example of a circadian rhythm?

Show answer

Answer

Our sleep-wake cycle is an example of a circadian rhythm. 

Show question

Question

How does the circadian rhythm affect you during the day?

Show answer

Answer

Whilst awake, you will use more resources to fuel your body. This includes exercise, walking, eating, and even thinking. The more complex the mental and physical tasks, the more resources you need to consume to fuel them. The digestive system also reacts accordingly during waking times compared to sleep times; when you sit down to have a meal the digestive system is already pre-emptively preparing for the meal beforehand by secreting and producing certain proteins and hormones.  

Show question

Question

How does the circadian rhythm affect you during the night whilst sleeping? 

Show answer

Answer

Whilst asleep, the body goes into a resting state and multiple functions are slowed down, such as your respiration rate, heart rate, and metabolic rate. Your body gets cooler during the night, due to the core temperature dropping during your deepest phases of sleep.  

Show question

Question

What did Siffre (1975) find in their study on circadian rhythms and lack of natural light?

Show answer

Answer

The natural sleep-wake cycle of 24 hours was extended to 25-30 hours during his time in the cave, demonstrating the necessity of light in keeping this system within its specific parameters. Whilst the circadian rhythm doesn't completely rely on the exogenous zeitgeber, it works alongside it to fine-tune it. 


Show question

Question

What are infradian rhythms?

Show answer

Answer

Infradian rhythms are biological rhythms that operate in a cycle lasting longer than 24 hours. They occur typically less than once per day and can cover weeks, months, and even years.  

Show question

Question

What is an example of an infradian rhythm?

Show answer

Answer

The menstrual cycle is a monthly infradian rhythm regulated by hormones. 

Show question

Question

What did Sabbagh and Barnard (1984) find regarding the infradian rhythm in women?

Show answer

Answer

Sabbagh and Barnard (1984) found that women's cycles synced up somewhat when they lived together, but why this is the case is not truly known (some hypothesise it is related to pheromones, but that's up for debate).  

Show question

Question

What are ultradian rhythms?

Show answer

Answer

Ultradian rhythms are cycles shorter than 24 hours. Ultradian rhythms have a shorter period and a higher frequency than circadian rhythms, occurring more than once every 24 hours.  

Show question

Question

What is an example of an ultradian rhythm?

Show answer

Answer

Sleep stages - the frequency and amplitude of the sleeping stage during the sleeping process. You will often cycle through various stages of sleep throughout the night, varying from light sleep to deep sleep and REM sleep.  

Show question

Question

What did Yang and Schank (2006) find regarding their study of women and menstrual cycles?

Show answer

Answer

They found that 186 Chinese women living together for a year did not synchronise their cycles over the year period. They also reviewed previous literature on this phenomenon and found that synchronisation and cycle variability may be due to various reasons, even pure chance and convergences, which were misinterpreted as synchronisation.  

Show question

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