Circadian rhythms

Summary

This lesson in biopsychology delves into the intricacies of biological rhythms, distinguishing between Circadian, infradian, and ultradian rhythms. It explores the interplay of endogenous and exogenous factors in regulating the sleep-wake cycle, with a focus on the role of the suprachiasmatic nucleus and melatonin. The lecture examines the implications of these rhythms on health and behavior, including the effects of desynchronization in shift workers and the use of light therapy for seasonal affective disorder. It also highlights the individual variability in circadian rhythms and the methodological challenges in studying them.

Takeaways

• 🕒 Biological rhythms are periodic cycles that occur at regular frequencies, with Cadian rhythms happening once every 24 hours, infradian less than once, and ultradian more than once.
• 🌓 Endogenous refers to internal body clocks, while exogenous refers to external factors that influence our rhythms, such as light, social cues, and meal times.
• 🔄 Entrainment is the synchronization of external and internal factors, highlighting the interaction between nature and nurture in regulating biological rhythms.
• 🌡️ Core body temperature is an example of a Cadian rhythm, with the lowest at 4 a.m. and the highest at 6 p.m., affecting alertness and cognitive ability.
• 💉 Hormones like cortisol follow a Cadian rhythm, peaking in the morning to help with stress response and alertness, and lowest in the middle of the night.
• 🌙 Melatonin and growth hormone peak at midnight, aiding in body restoration and growth, demonstrating the importance of sleep for overall health.
• 🛌 The sleep-wake cycle is a focus of the lesson, questioning whether our natural cycle is 24 hours or influenced by societal norms and external cues.
• 🧠 The superchiasmatic nucleus (SCN) in the hypothalamus is crucial for the sleep-wake cycle, receiving light information and signaling the pineal gland to produce melatonin.
• 🐹 Studies on animals, such as transplanting the SCN in hamsters and removing it in chipmunks, show the evolutionary importance and internal regulation of Cadian rhythms.
• 🕳️ The 'caveman' study, where an individual lived in isolation without external cues, revealed a natural tendency towards a slightly longer than 24-hour cycle.
• 💡 Light is a significant exogenous factor, affecting the sleep-wake cycle even when not perceived through the eyes, with implications for phototherapy and treating conditions like Seasonal Affective Disorder.

Q & A

What are the three main types of biological rhythms mentioned in the script?

The three main types of biological rhythms are Circadian, infradian, and ultradian rhythms. Circadian rhythms occur once every 24 hours, infradian rhythms occur less than once every 24 hours, and ultradian rhythms occur more than once every 24 hours.

What does the term 'endogenous' refer to in the context of biological rhythms?

Endogenous refers to the internal body clock, which is the internal mechanism within an organism that influences its biological rhythms.

What is the role of the exogenous factors in the sleep-wake cycle?

Exogenous factors are external to the body and can influence the sleep-wake cycle. These factors include the time of day, the presence or absence of light, social cues, and meal times.

What is the concept of entrainment in relation to biological rhythms?

Entrainment refers to the synchronization of external and internal factors. It is the process by which an organism's internal clock becomes aligned with external cues, such as light and dark cycles.

Can you provide an example of a Circadian rhythm other than the sleep-wake cycle?

An example of a Circadian rhythm is the core body temperature, which has its lowest point around 4:00 a.m. and its highest around 6:00 p.m.

What is the significance of the hormone cortisol in relation to the sleep-wake cycle?

Cortisol is a hormone that helps in the stress response and alertness. It is lowest in the middle of the night and starts peaking in the morning, preparing the body to deal with the day's demands.

What is the role of the superchiasmatic nucleus (SCN) in the regulation of the sleep-wake cycle?

The superchiasmatic nucleus (SCN) in the hypothalamus is responsible for receiving light information from the eyes and sending signals to the pineal gland to produce melatonin, which induces sleepiness in the absence of light.

What did the study involving the transplantation of the SCN in hamsters reveal about the role of the SCN in Circadian rhythms?

The study showed that the SCN is the main initiator of the Circadian rhythm, as the transplanted SCN from hamsters with a 20-hour sleep cycle into normal hamsters resulted in the 20-hour cycle being adopted by the recipients, demonstrating the SCN's influence over the rhythm regardless of external factors.

What was the outcome of the study involving the removal of the SCN in chipmunks?

The removal of the SCN in chipmunks resulted in a complete disruption of their Circadian rhythm, leading to irregular sleep patterns and an increased vulnerability to predators, highlighting the evolutionary importance of maintaining natural biological rhythms.

What are some practical applications of understanding the effects of endogenous and exogenous factors on the sleep-wake cycle?

Practical applications include using light therapy for treating Seasonal Affective Disorder, synchronizing medication schedules with the body's rhythm for better pharmacokinetics, and using melatonin supplements to help blind patients establish a 24-hour sleep-wake cycle.

What are some methodological issues or considerations when evaluating studies on biological rhythms?

Methodological issues include the individual differences in Circadian rhythms, the lack of population validity due to unique cases like the cave study, and the fact that conclusions drawn may not be reliable over time or across individuals due to factors like age and personal preferences.

Outlines

00:00

🌓 Understanding Biological Rhythms and Sleep-Wake Cycle

This paragraph introduces the concept of biological rhythms, focusing on the differences between Cadian, infradian, and ultradian rhythms. It explains that Cadian rhythms occur about every 24 hours, infradian rhythms less frequently, and ultradian rhythms more frequently within a 24-hour period. The paragraph also discusses the impact of endogenous pacemakers, such as the body's internal clock, and exogenous factors like light and social cues on the sleep-wake cycle. The concept of entrainment, which is the synchronization of external and internal factors, is introduced as a key to understanding the regulation of these rhythms. Examples of Cadian rhythms include the body's core temperature and hormone levels like cortisol, which fluctuate over a 24-hour period, affecting cognitive ability and stress response.

05:02

🧬 Internal and External Factors in Sleep-Wake Cycle Regulation

This paragraph delves into the internal and external factors that regulate the sleep-wake cycle, emphasizing the role of the hypothalamus and the superchiasmatic nucleus (SCN) in receiving light information and influencing the production of melatonin, a hormone that induces sleepiness. It highlights studies involving animals with genetic mutations affecting their sleep cycles and the transplantation of the SCN, demonstrating the SCN's role in maintaining Cadian rhythms. The paragraph also discusses the consequences of removing the SCN in chipmunks, leading to disrupted rhythms and highlighting the evolutionary importance of maintaining natural biological rhythms. The 'caveman' experiment is mentioned, showing that even without external cues, the body tends to adapt to a rhythm, suggesting an innate cycle that may be slightly longer than 24 hours.

10:05

💡 The Impact of Light and Other External Factors on Biological Rhythms

This paragraph explores the impact of light as a primary exogenous factor on the sleep-wake cycle, noting that light exposure can affect the cycle even without passing through the eyes, as demonstrated by studies applying light at the back of the knees. It also discusses experiments involving the manipulation of clocks to alter the cycle, suggesting that internal factors tend to override external ones. The paragraph emphasizes the significance of light in phototherapy and its potential application for blind individuals to adapt to a rhythmic cycle. It also touches on the limited evidence for other external factors significantly changing biological rhythms, indicating that the body has a natural tendency to maintain a rhythm.

15:06

🛌 Applications and Methodological Issues in Biological Rhythm Research

The final paragraph discusses the practical applications and implications of the research on biological rhythms, including the treatment of insomnia with melatonin supplements for blind patients and light therapy for seasonal affective disorder. It also addresses the methodological issues in the studies, such as the individual differences in Cadian rhythms and the lack of universality in the findings due to factors like age and unique cases like the 'caveman' study. The paragraph concludes by cautioning against drawing universal and reliable conclusions over time and across individuals, given the variability in cycle lengths and the influence of individual differences.

Keywords

💡Biological Rhythms

Biological rhythms refer to the periodic cycles that occur in living organisms, often at regular frequencies. In the context of the video, these rhythms are crucial for understanding the sleep-wake cycle and other physiological processes. The script discusses how these rhythms can be categorized into Circadian, infradian, and ultradian rhythms, each with different frequencies of occurrence.

💡Circadian Rhythms

Circadian rhythms are biological processes that follow a 24-hour cycle, aligning with the day-night pattern. The video script emphasizes Circadian rhythms as a key focus, especially in relation to the sleep-wake cycle. An example from the script is the core body temperature, which has its lowest point at 4:00 a.m. and highest at 6:00 p.m., illustrating the impact of Circadian rhythms on cognitive ability and alertness.

💡Infradian Rhythms

Infradian rhythms are those that occur less frequently than once every 24 hours. The script introduces this concept as a category of biological rhythms distinct from Circadian rhythms. Although not elaborated upon with specific examples in the script, infradian rhythms are relevant to understanding variations in biological processes that extend beyond a daily cycle.

💡Ultradian Rhythms

Ultradian rhythms happen more frequently than once every 24 hours. The video script mentions ultradian rhythms as another category of biological rhythms, contrasting with both Circadian and infradian rhythms. The script indicates that examples and further discussion of ultradian rhythms will be provided in a subsequent lesson.

💡Endogenous Pacemaker

An endogenous pacemaker refers to an internal clock within an organism that regulates its biological rhythms. The video script explains that this internal clock, such as the superchiasmatic nucleus (SCN) in the hypothalamus, is responsible for the sleep-wake cycle and is influenced by the presence or absence of light, a key factor in hormonal release related to sleep.

💡Exogenous Factors

Exogenous factors are external influences that can affect an organism's biological rhythms. The script discusses various exogenous factors such as the time of day, light exposure, social cues, and meal times that can influence an individual's rhythms. The role of light in affecting the sleep-wake cycle is highlighted, demonstrating the impact of external environmental factors on internal processes.

💡Entrainment

Entrainment is the synchronization of external and internal factors to align biological rhythms with the environment. The script explains entrainment as the interaction between nature (internal factors) and nurture (external factors), which is essential for maintaining a balanced rhythm. The concept is crucial for understanding how organisms adapt their rhythms to environmental changes.

💡Cortisol

Cortisol is a hormone associated with stress response and alertness. The script mentions cortisol as an example of a Circadian rhythm, with its levels lowest in the middle of the night and peaking in the morning. This hormone's pattern illustrates the body's preparation for the day's demands and its role in the sleep-wake cycle.

💡Melatonin

Melatonin is a hormone that helps regulate sleep by inducing sleepiness. The video script describes melatonin's peak production at midnight, aiding in the body's restoration and growth during sleep. The role of melatonin in the sleep-wake cycle is tied to the endogenous pacemaker and the external factor of light exposure.

💡Superchiasmatic Nucleus (SCN)

The superchiasmatic nucleus, or SCN, is a part of the hypothalamus that plays a central role in the regulation of Circadian rhythms. The script explains how the SCN receives light information from the optic nerves and influences the pineal gland to produce melatonin, thereby affecting the sleep-wake cycle. The SCN is highlighted as a key component in the internal regulation of biological rhythms.

💡Desynchronization

Desynchronization occurs when there is a misalignment between external and internal factors affecting biological rhythms. The script provides real-world examples such as shift workers and the consequences of this misalignment, such as reduced concentration and increased health risks like heart disease. Desynchronization underscores the importance of entrainment for optimal functioning and well-being.

Highlights

Biological rhythms are periodic cycles with different types: Circadian, infradian, and ultradian.

Circadian rhythms occur once every 24 hours, while infradian and ultradian rhythms have different frequencies.

Endogenous pacemakers are internal body clocks, and exogenous factors are external influences on these rhythms.

Entrainment is the synchronization of external and internal factors affecting biological rhythms.

Circadian rhythms, such as core body temperature and hormone levels, have a significant impact on cognitive ability and stress response.

The sleep-wake cycle is a focus of Circadian rhythm studies, questioning whether it is naturally 24 hours or influenced by societal norms.

The superchiasmatic nucleus (SCN) in the hypothalamus plays a crucial role in the sleep-wake cycle and is influenced by light.

Transplantation studies with hamsters show that the SCN is a primary initiator of Circadian rhythms, overriding external factors.

Desynchronization of internal and external factors can lead to disrupted biological rhythms and has real-world consequences.

Light exposure, even without passing through the eyes, can impact the sleep-wake cycle, which has implications for phototherapy.

Experiments with clock manipulation show that internal factors tend to override external factors in maintaining rhythm.

Desynchronization can lead to health issues such as reduced concentration and increased risk of heart disease among shift workers.

Melatonin supplements can help blind patients establish a 24-hour cycle, compensating for the lack of light perception.

Light therapy is beneficial for Seasonal Affective Disorder patients, showing the importance of light exposure across the body.

Pharmacokinetics can be optimized by synchronizing drug administration with the body's natural rhythms.

Individual differences in Circadian rhythms, such as age and personal tendencies, affect the generalizability of rhythm studies.

The study of biological rhythms and their regulation has practical applications in treating insomnia, optimizing medication effectiveness, and managing Seasonal Affective Disorder.

Methodological issues in rhythm studies include the validity of case studies, individual differences, and the impact of age on rhythm length.

Transcripts

so this is a lesson on biopsychology

biological rhythms for biological

rhythms you need to know the difference

between Cadian infradian and ultradian

rhythms you also know have to know about

the effect of endogenous pacemakers and

exogenous cabers on the sleep wake cyle

so we're going to look at those key

words to begin

with so biological rhythms are periodic

Cycles it means that they happen at a

regular frequency Cadian rhythms happen

once every 24 hours Circa means about

every 24 hours Al tradan means more than

once in 24 hours and infradian means

less than once every 24 hours we'll look

at examples of ultradian and infradian

rhythms in the next lesson endogenous

means inside it's the internal body

clock whereas exogenous means outside so

factors outside are body clock that

might influence our rhythms it could be

uh the time of the day the presence or

absence of light um social cues meal

times all these are factors that might

influence and lastly entrainment is not

a named key word but it's concept that

means basically the

synchronization of external and internal

factors as usual when we look at these

nature nurture questions nature internal

factors Nur external factors it always

ends up being an inter interaction

between the two and that's what

entrainment basically

means so although the focus is on the

sleep wake cycle Cadian rhythms are

mentioned so they might ask you to give

more than one example of Cadian rhythms

so we're briefly going to look at two

more examples of Cadian rhythms one

example is core body temperature the

lowest body temperature happens at 400

a.m. the highest at 6:00 p.m. and then

we have a dip after lunch if you notice

that you're a bit less efficient less

alert after lunch that might be because

of a drop in body temperature it's

linked to co cognitive ability so it

makes sense that in the middle of the

night we have our lowest cognitive

ability because we don't need to use

it a second example of Cadian rhythm is

hormones cortisol helps in stress

response and alertness and it's it's its

lowest again in the middle of the night

and then it starts peaking in the

morning when we start having to deal

with the stress of just starting a new

day or the kind of Demands of the new

day melatonin and the growth hormone

peak at midnight because actually

they're helping our body restore grow de

and kind of fix any issues that have

happened during the day so these work in

a cyclical pattern over 24 hours as

[Music]

well so the sleep wake cycle is the

Circadian rhythm that we're going to be

focusing on that is named on the

syllabus and the kind of questions that

you might be asked to think about is

whether our natural sleep wake cycle is

actually 24 hours or is that to do with

society that has kind of arbitrarily

decided that a day lasts 24 hours how

would our body work if it was on kind of

free flow without any social cues what

conditions could make you wake up

earlier or later what might make you

sleep longer or

shorter and therefore the focus of this

lesson will be what regulates the sleep

wake cycle and to what extent is it

endogenous pacemakers internal factors

that regulate the sleep wake cycle is it

more to do with exogenous zabas the

external factors as usual the conclusion

is never one way or another but it's

combination of the two but the studies

that we'll look at focus on trying to

understand and answer this question by

looking at what would happen if we

remove or external factors what would

happen if we remove internal factors and

to what extent that influences the sleep

wake cycle and that would be how you

would develop your answer just as a

quick guideline from experience from

when this has come up as an essay

question you need to think about what

would you use as ao1 and AO3 now

typically ao1 will be discussing as you

can see here the extent to which we know

about influences of endogenous factors

and to what extent we know about the

influence of exogenous factors so there

are studies that we're going to look at

uh Cave studies that are people that

isolate themselves from external factors

so obviously this is looking at to what

extent the internal factors Run free if

you remove all external factors and then

there's studies on surgeries that have

been used to uh remove parts of the

brain that seems to be responsible for

Cadian rhythms and Transplant them in

other animals to see whether those

Cadian rhythms change therefore for

again proving that internal factors

override external factors on the other

hand in uh the effects of external

factors there are experiments using

light on um participants to see to what

extent exposure to light can impact on

the sleep wake cycle and also uh

experiments where the clocks have been

changed sped up to see whether that has

an impact on the sleep wake cycle so

your ao1 is looking at the effect of

endogenous and exogenous Pacemakers in

which case your evaluation will be more

on the line of real world applications

real world examples practical

applications uh in uh for example

treating insomnia pharmacokinetics that

will come back to uh and also evaluation

so methodological evaluation of studies

especially the fact that there are

individual differences in the Cadian

rhythms of of people and therefore any

study can't necessarily be generalized

to the entire

population so for A1 we're going to look

at these key studies on endogenous and

exogenous pacemakers endogenous

pacemakers first of all you would look

at the biology underlying the sleep wake

cycle there's a part in the hypothalamus

called the super chiasmatic nucleus that

meets at the crossover of the two optic

nerves and it receives from the eyes

information about whether it's light or

not now once that happens the supermatic

nucleus that you can called SC you don't

have to write the entire word it sends

information to the pineal gland that

then if there is no light then it will

start producing melatonin and melatonin

induces sleepiness so the conclusion is

that the natural currence of the sleep

wake cycle is determined by hormonal

factors however already we have to

acknowledge that that hor hormone or

release is triggered by the presence or

absence of light that would be an

exogenous sight

Gaber as I mentioned we've got studies

on animals where the scn was

transplanted from one hamster to the

other so this Morgan study was found

that there was some hamsters that had a

genetic mutation whereby they had a 20

hour sleep cycle rather than 24 hours so

what they did is they took the scn the

supermatic nucleus of the mutant

hamsters and transplanted it in normal

wild type hamsters and what they found

is that in fact the mutant uh period the

20-hour cycle uh was transplanted also

into the wild type h hamsters showing

that the scn does seem to be the

main initiator of the Cadian Rhythm

regardless of external factors because

the transplant of that part of the brain

did result in a change in Cadian Rhythm

we've also got uh the deos chipm study

and this was the removal of supermatic

nuclei in Chipmunks so that meant that

uh they lost their internal Pacemaker

and what that resulted in you can uh

guess was that their Cadian Rhythm

became completely disrupted they were up

up in the night sleeping in the day and

this shows how evolutionary important it

is to have natural biological rhythms

not only dictated from the outside

because actually sadly a lot of these um

Chipmunks died because of predators

catching them because they were out and

about at inappropriate times so there is

an evolutionary basis of the need for

Cadian Rhythm and that can kind of

justify why internal natural factors are

more important than external

factors s

is a famous um caveman he named himself

the caveman because he spent a long

amount of time all by himself 6 months

as you can see in a cave with no natural

light no clocks no television no radio

basically he had no clue as to what day

it was whether it was day or night and

whether it was time to sleep or not so

he decided himself what to do and when

on the basis of what his body was

telling him again an example of how

isolated external factors can it help us

explain whether internal factors

override everything there are

contrasting interpretations here of the

findings because what we found is that

he did adapt to a slightly longer Cadian

rhythm of 25 hours uh because when he

came out he thought it was a month

earlier than it was because his days

were kind of prolonged in his head

however it does show that he still got

into a kind of a rhythm and a cycle so

it can be interpreted in two ways one

way is our natural cycle is not 24 hours

so the fact that it's 24 hours is

because of external cues Demand on the

other hand that we do have a tendency to

work at a certain

Rhythm um now we're moving on to

external factors so that light is the

main ex exogenous zit Gaber and the

effects of light in this case was

studied by looking at to what extent

light has an effect on us even if it

doesn't pass through the eyes this is

significant because if it doesn't come

through Through The Eyes it's not

necessarily communicated to the scn and

what was found here is that light

applied at the back of the knees of

individuals still impacted on their

sleep wake cycle by about 1 to three

hours shifting it so waking them up

basically meaning that exposure to light

happens regardless of whether it happens

Through The Eyes this is very

significant in terms of phototherapy in

terms of how blind people can adapt uh

to a kind of cycle rythm even if they

can't perceive light through their

eyes so it shows the importance of

light this was the study I mentioned

before they another one on exogenous

light gers this time it's not light but

it's the clocks they had 12 people live

in isolation again so no cues at all no

idea what time it was except for the

clocks that were in the in the rooms and

they tried to change the the the timing

of the clocks so they would go fast and

they would run on a 22h hour cycle

rather than 24hour and actually although

participants initially uh seemed to

match the clocks eventually as it got

quicker they didn't manage to keep up

suggesting again that internal factors

did seem to override external factors so

besides light there's not strong

evidence that external factors can

actually significantly change our

biological Rhythm and that our body body

does have a tendency to want to

maintain uh a

rhythm so now we're going to move on to

the A3 we're going to look at

applications implications so the

consequences of the reearch and actual

issues with research that you can um

comment on in your AO3 if you present

your research in ao1 okay so first

implications are um desynchronization so

there is real world evidence that when

the external factors and the internal

factors are desynchronized like in the

Chipmunks where they they remove the the

supermatic nucleus and most of them a

lot of them died we have real world

evidence in the human world as well

meaning you can't say animal studies

don't generalize because here we have

the same findings shift workers

experiened reduced concentration at 6:00

a.m. and this is real world because cob

and the free mile island that were both

nuclear disasters happened in the middle

of the night EXP explaining that

probably because people are not supposed

to be awake at night their cognitive

ability is much decreased and it was

because of human era that these

disasters happened we've also got

evidence that shift workers are three

times more likely to develop heart

disease probably because they have to

put extra strain on their body to remain

awake when they're supposed to be

sleeping so this shows us definitely

that the interaction between external

and internal factors is that entrainment

where they matched is the optimal

condition for

survival we've also got uh practical

applications as I mention for insomnia

in blind patients it was found that uh

giving melatonin supplements to these

patients actually helped them to get

into a 24-hour cycle so because the the

light wasn't perceived by the supermatic

nucleus by the SC and therefore

melatonin might not have been produced

taking me melatonin supplement

actually compensated for for that

absence so that shows that still even

though light has an effect uh reducing

uh insomnia is linked to taking

supplements that reproduce that scn

pineal gland and melin

cycle we've also got implications for

light therapy and that's especially for

Seasonal effective disorder patients

that experience depression in in Darker

nights this just shows that we can shine

light on on individuals not only through

their eyes but across their whole whole

body and that should decrease the

symptoms of Seasonal effective

disorder um pharmacokinetics is another

example of practical application and

that's when uh we synchronize the body

Rhythm whe with the time that the drugs

are taken we optimize their

effectiveness because if the drug is

taken regularly then then it becomes

part of the biological Rhythm meaning

that it might will respond and kind of

syn up with the body much more

effectively so our knowledge of Cadian

rhythms and the effect of endogenous and

exogenous pacemakers doesn't only exist

in theory but actually has an impact on

the well-being of people with insomnia

people taking medication and people with

Seasonal effective

disorder lastly as I mentioned we're

going to talk about methodological

issues the first one is that for example

cph was a case study so we could argue

that it's a unique case and it lacks

population validity we do however have a

replication uh in the same conditions

there's these a series of studies by

ashro and Weaver in uh these World War

Two Bonkers in Germany where they had

lots of volunteers uh going into these

Bonkers again with no external cues and

they found quite similar results

everybody adapted to slightly longer

side than the 24 hour cycle so agreeing

with those conclusions that it is an

interaction between internal and

external factors that result in that

24-hour cycle however there were some

that had Cycles as long as 28 hours

again highlighting the matter of

individual differences and the lack of

population validity regardless of the

size of the sample because we don't have

consistent

outcomes in terms of individual

differences you can mention the L Al

test dafal developed the questionnaire

you can take it you can find it online

that basically shows that some people

morning people some people are evening

people so not everybody's rhythm is

exactly synced with the time of the day

and also the length of the cycle can

vary from person to person another

factor that CH changes our cycle is age

for example the the amount of sleep that

we get H redu significantly as we grow

older and also CRA himself repeated his

study um about 25 years later when he

was much older and he found that his

clock was also much slower than he was

younger so we can't draw conclusions

that are Universal and even if they were

Universal they're not uh reliable over

time and across individuals so we need

to draw conclusions with some caution