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Biological Rhythms

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Biological Rhythms

Have you ever wondered how your body knows when to go to sleep and when to be awake? Have you noticed that you feel more alert at certain times of the day than others? Maybe you have even noticed that you feel different depending on the weather or the time of the year/position of the earth. Each of these examples is controlled by your body's biological rhythms!

Biological rhythms definition

Biological rhythms, also known as biorhythms, endogenous rhythms, internal rhythms or life rhythms, are natural, biological events or functions occurring in living organisms. Biological rhythms follow repetitive patterns determined by internal and environmental changes. Human biological rhythms include the sleep-wake cycle and the menstruation cycle.

The sleep-wake cycle is an example of a biological clock (circadian rhythm). Repetition of intervals between the events allows the organism to harmonise with its environment. Our internal biological rhythms create physiological changes.

Human bodies react to the earth's rotations in relation to the sun and the moon, making us feel sleepy when the sun goes down. This alternation between day and night creates human biological rhythm changes.

Biological Rhythms, Clock Day and Night create biological changes, StudySmarter

Biological rhythms circadian, freepik.com/redgreystock

Key factors that influence biological rhythms in the human body

Two key factors determine biological rhythms:

  • The external body clocks, or exogenous zeitgebers, are affected by environmental changes.

  • The internal clocks, or endogenous pacemakers, are directly affected by internal, physiological factors (typically these are genetically determined).

You can read more about biological rhythms and bases of thinking, behaviour, and emotion in 'Biopsychology'.

A good example of an endogenous pacemaker is the suprachiasmatic nucleus (SCN).

The SCN is a structure located in the hypothalamus and acts as an internal clock for the circadian rhythm system within your body.

It helps maintain the sleep-wake cycle by detecting changes of light in your environment based on photosensitive cells within the retina of your eyes. The SCN innervates (stimulates through the nerves connected) the pineal gland based on this information, which produces the key hormone melatonin.

When there is less light, higher levels of melatonin are produced which encourages the body to sleep. Conversely, if there is more light, less melatonin is produced which usually occurs during the hours you are 'supposed' to be awake.

A lot of medical researchers within the last few years have highlighted some concerning issues with the encroaching problem of increased screen time and light exposure modern society faces. The number of people spending more time looking at a screen increases. Screen time at night whilst you lay in bed has a large impact on the natural sleep-wake cycle within your body.

The circadian system essentially operates on a 24-hour cycle, in that these cycles occur once every 24 hours. It typically depends on the feedback given by your body on how long you have been awake, which relies on the body's homeostasis mechanisms.

If you are awake for too long, the sleep-wake cycle will detect this and signal the need for sleep, and this reservoir of a rising need to sleep builds up over the course of the day until it reaches its maximum capacity.

Despite our endogenous pacemakers maintaining the genetically determined mechanism of sleep within our body, we are still able to choose when to sleep and when to be awake.

Exogenous zeitgebers work in collaboration with your endogenous pacemakers. They use outside, external factors as a prompt to trigger different biological rhythms within your body, including the one for sleep.

Siffre (1975) spent isolated time in a cave, where there was no ability to keep the time, nor the ability to rely on natural light to estimate the time, either. He wanted to understand how his sleep-wake cycle was affected in the absence of these time-keeping devices. Interestingly, Siffre's sleep-wake cycle changed to a 25-30 hour cycle, instead of the 24-hour one. This implies that whilst the circadian rhythm is mostly able to establish a 24-hour cycle, it works in tandem with exogenous zeitgebers to maintain and fine-tune it.

The four types of biological rhythms

Overall, there are four biological rhythms within the body. These include circadian, diurnal, infradian, and ultradian rhythms.

Circadian rhythms

As we established above, circadian rhythms are a biological version of a clock inside humans, animals, plants and possibly almost all living cells, and occur once every 24 hours. These rhythms primarily rely on light to operate and keep the 24-hour cycle. Daylight regulates the clock cycles comprising one day and one night.

Diurnal rhythms

Diurnal rhythms are circadian rhythms that have synced to occur primarily during daylight hours. The circadian rhythm period may vary beyond 24 hours. When the rhythm is synchronised with the day and night cycle, it is called a diurnal rhythm.

Body temperature and heart rate vary throughout the day in sync with the natural day and night cycle.

Infradian rhythms

Infradian rhythms are biological rhythms that operate in a cycle lasting longer than a 24-hour period. Its rhythm patterns recur weekly, monthly or annually. The female menstrual cycle is a monthly infradian rhythm regulated by hormones.

Ultradian rhythms

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

The sleeping stages change in frequency during the sleeping process, i.e. REM sleep, deep sleep, and light sleep. Having a regular cycle of these stages is incredibly important for health in general, and disruption can cause various issues.

Biological Rhythms, a man suffering from insomnia, StudySmarterInsomnia disrupting sleep cycle, freepik.com/storyset

Disruption of biological rhythms can cause a whole host of issues, including slowing reaction times, affecting our cognitive skills such as problem-solving skills, and affecting our ability to focus on things. You may have felt this when you are tired because you didn't get enough sleep on a school night. Trying to focus during maths is a hard task when your brain wants to sleep.

Cziesler et al. (1982) found that those with sleep patterns affected by their work situations, such as working day and night shifts without time to adjust properly, had health and sleep problems. Cziesler et al. (1982) suggested that if a working pattern that took the circadian rhythm into account was established, it would improve health and sleep. They implemented something called phase delay and a 21-day shift pattern. This resulted in increased productivity and job satisfaction.

More on circadian rhythms

Circadian rhythms directly affect all living organisms. They support the processes that are optimised according to different moments of the day or during a 24-hour cycle, coordinating mental and physical systems throughout the body.

Circadian rhythms are physiological and behavioural rhythms and they regulate our sleep-wake cycle.

They respond to natural cycles of light and dark and reset the organism via the levels of light received. When the retina perceives light it triggers a response, alternating the levels of hormones excreted or body temperature within specific periods for optimum energy expenditure.

However, that's not to say all living organisms are bound to a circadian rhythm. Rather, they have a biological rhythm that differs depending on the goal they are trying to achieve.Our internal biological rhythms create periodic physiological fluctuations.

The human biological rhythms

The human biological rhythms are:

  • Sleep-wake cycle

  • Body temperature

  • Hormone secretion patterns

  • Blood pressure

  • Digestive secretions

  • Alertness

The biological clock and the circadian rhythm

The biological clock includes circadian rhythms or activities synchronised with an environmental stimulus. Circadian rhythms are connected to the body and a master clock, or internal clock, that is located in the brain. It synchronises the alternation between cycles of alertness and sleepiness.

The biological clock is the internal mechanism that supports regulating our body’s timing processes.

The circadian rhythms align with our sleep and wakefulness between day and night.

It creates a stable and harmonious cycle of rest that allows us to have more energy to be active during the day. This biological circadian system helps humans adapt to changes in the environment and allows us to anticipate changes in our environment like radiation, temperature and food opportunities.

The biological clocks affect the circadian rhythms, but not all biological clocks are circadian.

The circadian rhythms become more evident when looking at the sleep-wake cycle, emphasising its importance.

Plants, for example, adjust themselves to changes related to the seasons or the biological clock. However, the process lasts beyond the 24-hour cycle, which is longer than the circadian cycle.

Issues with studying biological rhythms

It is hard to truly study the biological rhythms, in that studying animals has generalisability issues, and studies removing natural light still allowed for artificial light, reducing the validity of their findings. There are also issues with individual differences, in that people can be 'morning people' and 'evening people'.

Some find it easy to wake up in the morning and have an average of six hours of sleep, whilst others struggle immensely with early morning starts and struggle to fall asleep early at night.


Biological Rhythms - Key takeaways

  • Biological rhythms are natural, biological events or functions occurring in living organisms that follow repetitive patterns determined by internal factors and environmental changes.
  • The four types of biological rhythms are circadian, infradian, diurnal, and ultradian rhythms.
  • Circadian rhythms are biological rhythms that last 24 hours and are commonly known as the 'biological clock', which is reset by levels of light.
  • Diurnal rhythms are circadian rhythms that have synced to occur primarily during daylight hours.
  • Infradian rhythms are biological rhythms that operate in a cycle lasting longer than a 24-hour period, and ultradian rhythms are cycles shorter/occur more than once every 24 hours.

Frequently Asked Questions about Biological Rhythms

Biological rhythms are natural, biological events or functions that occur in living organisms. Biological rhythms follow repetitive patterns responding to a period of environmental changes by keeping repetitive intervals between events. This allows the organism to harmonise within its environment. Our internal biological rhythms create physiological changes.

Two key factors determine biological rhythms. The external body clocks, or exogenous zeitgebers, are influenced by environmental changes. The internal clocks, or endogenous pacemakers, are directly affected by physiological factors. They influence the human body’s physical, mental and behavioural activities, the sleep-wake cycle and our temperature regulation.

Homeostasis is the ability to maintain the internal bodily environment in constant equilibrium through dynamic interactions in physiological and biochemical systems. The homeostasis mechanism and the circadian rhythm, which is one of the biological rhythms, collaborate to determine sleep-wake cycles. The homeostatic drive increases throughout the day; in the late evening, when most people need to sleep, it reaches its maximum capacity.

The circadian rhythm is an effect of a biological clock. Not all biological clocks follow a circadian rhythm, which has a 24-hour cycle. It works by regulating the alternation between cycles of alertness and sleepiness.

The internal biological rhythms create physiological changes, making us feel sleepy when the sunsets. When people are awake for a long time, the homeostatic physiology of the sleep-wake cycle sends negative feedback to the body to signal the need for sleeping to reset the energy levels. 

Final Biological Rhythms Quiz

Question

 What are diurnal rhythms?

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Answer

Diurnal rhythms refer to a rhythmic synchronisation within the day and night cycle.

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Question

How long do infradian rhythms last?

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Answer

Over 24 hours.


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Question

How long do ultradian rhythms take?



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Answer

 Ultradian rhythms are cycles shorter than 24 hours.

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Question

What are biological rhythms?



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Answer

Biological rhythms are natural biological events in living organisms 

that follow repetitive patterns determined by environmental changes.


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Question

What are the suprachiasmatic nuclei 

(SCN) responsible for?



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Answer

The SCN is connected to the pineal gland, 

and both are responsible for maintaining the 

circadian sleep-wake cycle process.

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Question

What are exogenous zeitgebers responsible for?  



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Answer

The exogenous zeitgebers are responsible for 

resetting the biological clock of an organism. 


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Question

What are the two biological functions 

that follow the circadian cycle?

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Answer

The two main human biological rhythms 

that follow the circadian cycle are our 

sleep-wake cycle and body temperature. 

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Question

What is the function of the “biological clock”?

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Answer

The biological clock is responsible for managing 

the synchronicity within the environmental stimulus.

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Question

What affects exogenous zeitgebers?

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Answer

The external body clocks, or exogenous zeitgebers, 

are affected by environmental changes.

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Question

How do the biological clock fluctuations activate the sleep-wake cycle?



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Answer

The biological clocks are present in every cell, which is synchronised by the (SCN) located in the hypothalamus. During sleep, some genes produce proteins that gradually increase throughout the night, lowering during the day. These fluctuations activate the sleep-wake cycle.

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Question

How long does a sleep-wake cycle last?




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Answer

The sleep-wake cycle has a recurring pattern that is synchronised with the 24-hour clock during which we spend time being awake and asleep.

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Question

What is the role of the sleep-wake cycle? 



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Answer

The role of the sleep-wake cycle is to regulate our sleep at night and keep us awake during the day.


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Question

What are the main mechanisms responsible for the sleep cycle?




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Answer

There are internal biological mechanisms responsible for our sleep cycle: circadian rhythm 

and homeostasis.

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What are the other names given to biological rhythms?

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Answer

Biological rhythms, also known as biorhythms, endogenous rhythms, internal rhythms or life rhythms.

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Question

How do biological rhythms affect our physiological body?

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Answer

Our internal biological rhythms affect our physiological body by creating changes.

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Question

What are exogenous zeitgebers?

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Answer

Exogenous zeitgebers are the key factors that manage the biological rhythms.

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Question

What are exogenous zeitgebers?

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Answer

Exogenous zeitgebers are the key factors that manage the biological rhythms.

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Question

What are Endogenous pacemakers?

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Answer

Endogenous pacemakers are internal 

mechanisms that control our biological rhythms.

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Question

Outline an example of endogenous pacemakers.



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Answer

An example of endogenous peacemakers is the circadian sleep-wake cycle.

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Question

What is homeostasis?


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Answer

Homeostasis is the ability to maintain the internal bodily environment 

in constant equilibrium through a series of dynamic interactions 

in physiological and biochemical systems.

Show question

Question

What are biological rhythms?

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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.  

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Question

What are endogenous pacemakers?

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Answer

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

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Question

What is the main endogenous pacemaker in mammals?

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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.  

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Question

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

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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. 

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Question

What is the circadian rhythm?

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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.  

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Question

What is an example of a circadian rhythm?

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Answer

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

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Question

How does the circadian rhythm affect you during the day?

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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.  

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

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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.  

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Question

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

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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. 


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Question

What are infradian rhythms?

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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.  

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Question

What is an example of an infradian rhythm?

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Answer

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

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Question

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

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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).  

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Question

What are ultradian rhythms?

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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.  

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Question

What is an example of an ultradian rhythm?

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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.  

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Question

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

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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.  

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