Biological Rhythms: Circadian Rhythms and endogenous pacemaker/ exogenous zeitgeber

Question 1

What are biological rhythms?

Answer 1

Distinct patterns of changes in body activity that conform to cyclical time periods e.g. circadian rhythms (once every 24 hours) infradian (each cycle lasts more than 24 hours) and ultradian rhythms (more than once every 24 hours). Biological rhythms are influenced by internal body clocks (endogenous pacemakers) as well as external changes to the environment (exogenous zeitgebers)

Question 2

What are circadian rhythms?

Answer 2

Biological rhythms that last approximately 24 hours- there are several type of circadian rhythms including the sleep/wake cycle, body temperature and hormone production

Question 3

What is the sleep/wake cycle?

Answer 3

A circadian rhythm that is influenced by regular variations in the environment, such as the alternation of light and day

Question 4

Sleep/wake cycle is governed by both endogenous pacemaker and exogenous zeitgebers e.g.

Answer 4

• exogenous zeitgebers=the fact that we feel drowsy when it’s night-time and alert during the day shows the effect of daylight
• endogenous pacemakers= if the biological clock is ‘left to its own devices’, without the influence of external stimuli such as light, it is called ‘free-running’. Research has shown that there is a basic rhythm governed by the suprachiasmatic nucleus (SCN). The SCN lies just above the optic chiasm (where optic nerves from each eye cross over) and receives information about light directly from this structure; thus the exogenous zeitgeber (light) can ‘reset’ the endogenous pacemaker- I.e. SCN can ‘run’ on its own time but in order to stay synchronised with the rest of the world, it needs external cues such as light to ‘reset’ it

Question 5

What are endogenous pacemakers?

Answer 5

Internal body clocks that regulate many of our biological rhythms such as the influence of the superchiasmatic nucleus (SCN) on the sleep/wake cycle

Question 6

What are exogenous zeitgebers?

Answer 6

External factors in the environment that reset our biological clocks through a process known as entrainment, such as the influence of light on the sleep/wake cycle

Question 7

Discuss role of endogenous pacemaker in sleep/wake cycle

Answer 7

• superchiasmatic nucleus (SCN) is one of the primary endogenous pacemakers often referred to as the ‘master clock’
• SCN is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep/wake cycle
• SCN is a tiny bundle of nerve cells in the hypothalamus in each hemisphere of the brain
• SCN lies just above the optic chiasm and receives information about light directly from this structure, which sets the circadian rhythm so that it is synchronised with with the outside world e.g. day and night

Question 8

Explain how the pineal gland and melatonin are endogenous mechanisms to bring on sleep

Answer 8

• SCN sends ‘signals’ (information on day length and light it receives due to its photoreceptors) to the pineal gland which leads to an increase in the production of melatonin- hormone produced by pineal gland that increases sleepiness and is inhibited in periods of wakefulness; in humans it is usually produced at night and regulated by the SCN) at night
• SCN and pineal glands work together as endogenous pacemakers however their activity is responsive to the external cue of light

Question 9

Melatonin has also been suggested as a causal factor in

Answer 9

Seasonal affective disorder (SAD)- depressive disorder associated with seasonal changes- usually the onset of winter and increased darkness. As with other forms of depression, the main symptoms are persistent low mood, and a general lack of interest and activity in life e.g. avolition

Question 10

Explain how light is a significant exogenous zeitgeber that influences the sleep/wake cycle

Answer 10

Light can reset the SCN= plays a key role in the sleep/wake cycle. If light of high intensity is detected by the retina, the production of melatonin is inhibited and so feelings of sleepiness and drowsiness are not induced = wakefulness

Question 11

Explain a study that suggests light is a powerful exogenous zeitgeber

Answer 11

• Campbell and Murphy (1998) demonstrated that light may be detected by skin receptor sites on the body even when the same information has not been received by the eyes
• 15 participants were woken at various times and a light pad was shone on the back of their knees
• researchers managed to produce a deviation in the participants’ sleep/wake cycle of up to 3 hours in some cases= suggests that light is a powerful exogenous zeitgeber that need to necessarily rely on the eyes to exert influence on the brain

Question 12

Explain how social cues have an important influence on the sleep/wake cycle

Answer 12

• infants are seldom on the same sleep/wake cycle as the rest of the family
• in human infants, the sleep/wake cycle is pretty much random
• at about 6 weeks, most babies are entrained- the schedules imposed by parents are likely to be a significant influence here, including adult-determined meal times and bedtimes
• research also suggests that adapting to local times for eating and sleeping (rather than responding to one’s own feelings of hunger and fatigue) is an effective way of entraining circadian rhythms and tackling jet lag when travelling long distances

Question 13

What’s a study to support social cues as exogenous zeitgeber?

Answer 13

One of the earliest studies on jet lag found that the circadian rhythms of air travellers adjusted more quickly if they went outside more at their destination; this was thought to be because they were exposed to the social cues of their new time zone, which acted as a zeitgeber

Question 14

What is a study which support the role of the SCN (endogenous pacemaker) in establishing and maintaining sleep/wake cycle?

Answer 14

Decoursey et al - destroyed SCN connections in brains of 30 chipmunks

Question 15

Explain the procedure and findings of Decoursey et al (2000)

Answer 15

• destroyed the SCN connections in the brain of 30 chipmunks who were then returned to their natural habitat and observed for 80 days
• the sleep/wake cycle of the chipmunks disappeared and a significant proportion were killed by predators (because they were awake and vulnerable when they should have been asleep)

Question 16

What do findings of Ralph and Decoursey suggest?

Answer 16

Emphasise the role of SCN I’m entraining and maintaining sleep/wake cycle

Question 17

What are the evaluation points for endogenous pacemaker and exogenous zeitgebers?

Answer 17

✅support studies for SCN = endogenous pacemaker = Ralph and Decoursey
✅ support study of socials cues = exogenous zeitgeber = earliest jetlag study
❌limitation of research into SCN is that it may obscure other body clocks
❌ use of animals- anthropomorphism
❌ influence of exogenous zeitgebers may be overstated
❌ endogenous pacemaker and exogenous zeitgeber interact

Question 18

Discuss how a limitation of research into SCN is that it may obscure other body clocks

Answer 18

Research has revealed there are numerous circadian rhythms in many organs and cells in the body- peripheral oscillators, found in the lungs, liver, pancreas and skin for example. Although these peripheral clocks are highly influenced by the actions of the SCN, they can act independently. Damiola et al (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver for up to 12 hours whilst leaving the rhythm of the SCN unaffected = may be other complex influences on the sleep/wake cycle aside from the master clock (SCN)

Question 19

Discuss how a limitation of research into endogenous pacemaker and exogenous zeitgebers is the use of animals

Answer 19

First and foremost, there is an issue generalising findings from research into the sleep/wake cycle from animal studies. For example, rats and mice are often used in sleep/wake cycle experiments like in Damiola et al’s experiment in 2010 but we know that due to humans being higher functioning organisms (e.g. ability of metacognition) and the physiological differences in brain structures (mouse cerebral cortex is less developed in contrast to the cerebral cortex of higher animals, including humans) means that we are unable to generalise findings to humans. Therefore, they do not contribute significant value to understanding the role of endogenous pacemakers and exogenous zeitgebers in humans.
Also, a more disturbing issue however, particularly in relation to the Decoursey et al study is the ethics involved in such research- the animals were exposed considerable harm, and subsequent risk when they were returned to their natural habitat. Ultimately, whether what we learn/gain from investigations such as these justifies the aversive procedures involved is a complex matter of debate

Question 20

Discuss how the influence of exogenous zeitgebers may be overstated

Answer 20

studies of individuals who live in Artic regions (where the sun does not set during the summer months) show normal sleep patterns despite the prolonged exposure to light. Both these examples suggest that there are occasions where exogenous zeitgebers may have little bearing on our internal rhythms

Question 21

Discuss how endogenous pacemaker and exogenous zeitgeber interact

Answer 21

Only in exceptional circumstances are endogenous pacemakers, free-running and unaffected by the influence of exogenous zeitgebers- total isolation experiments like Siffre’s cave study are extremely rare and could be judged as presenting an unrealistic picture of how the system works. In real life, endogenous pacemakers and exogenous zeitgebers interact and it may make little sense to separate the two for the purpose of research

Question 22

Explain how Siffre demonstrated a free-running circadian rhythm of about 25 hours

Answer 22

In a series of very unusual experiments, beginning in 1960s, Michel Siffre spent long periods of time in dark caves to examine the effects of free-running biological rhythms. Deprived of exposure to natural light and sound, but with access to regular food and drink, Siffre resurfaced in mid-September 1962 after 2 months in the caves of Southern Alps- a decade later he performed a similar but this time for 6 months in a Texan cave. In each case, Siffre’s free-running circadian rhythm settled down to one that was just beyond the usual 24 hours (25 hours), though he did continue to fall asleep and wake up on a regular basis

Question 23

What can be concluded from Siffre’s experiments?

Answer 23

1- the body generally keeps to a sleep-wake cycle without the use of external cues
2- without external cues this cycle can vary unpredictably

Question 24

Explain how Aschoff and Wever also found an similar circadian rhythm to Siffre?

Answer 24

A group of participants spent 4 weeks in a WW2 bunker deprived of natural light- all but one of the participants displayed a circadian rhythm between 24 and 25 hours

Question 25

Explain how Folkard et al’s study demonstrated that endogenous pacemakers has a stronger influence than exogenous zeitgebers

Answer 25

• conducted an experiment to establish whether external cues (exogenous zeitgebers) could override the internal body clock (endogenous pacemaker)
• Folkard et al studied a group of 12 people who agreed to live in a dark cave for 3 weeks, retiring to bed when the clock said 11:45 and rising when it said 7:45
• Over the course of the study, the researchers gradually speeded up the clock (unbeknown to the participants) so an apparent 24 hour day only lasted 22 hours
• it was revealed that only one of the participants was able to comfortably adjust to the new regime suggesting the existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in the external environment

Question 26

What are the evaluation points for Siffre?

Answer 26

❌ used artificial dim lighting to navigate his way around- aim was to remove all exogenous zeitgebers to examine the effects of free-running biological rhythms, however Siffre used dim artificial light to navigate around the caves and turned on a lamp every time he woke up until he went to bed. It was assumed that dim lighting would not interfere with the free- running biological rhythm. The use of dim lighting may be similar to taking a drug that resets their body clock. This suggests that researchers may have ignored an important confounding variable in circadian rhythm research, ultimately reducing the interval validity of the experiments As a result, it could be argued that all exogenous zeitgebers have not been eliminated and so the experiment wasn’t a true study of free-running biological rhythms reducing its internal validity

✅ further research support e.g. Aschoff and Wever

✅ scientific measures e.g. use of electrodes to monitor heart rate and temperature during his sleep/wake cycle= quantitative data obtained which cannot be disputed or vary between analysis by various scientists = reliable as objective data doesn’t rely upon subjective interpretation -generally qualitative data obtained with case studies which is subject to observer bias however this has been eliminated

Question 27

Discuss how body temperature is a circadian rhythm

Answer 27

• temp at its lowest at 4:30am and its highest at 6pm
• generally speaking, body temp decreases as we prepare for sleep
• conversely, body temp begins to rise after 4:30am in order to create a heightened alertness ready for waking up
• also a dip in temperature around 2-4pm, which is why you may feel a drop in energy shortly after lunchtime

Question 28

Discuss how hormone production is a circadian rhythm

Answer 28

• Many hormones, particularly cortisol and melatonin also work on a circadian rhythm
• melatonin is a hormone produced by pineal gland that increases sleepiness and is inhibited in periods of wakefulness; in humans it is usually produced at night and regulated by the SCN- melatonin production increases towards bedtime and decreases as the day progresses and light increases
• cortisol is a hormone which causes alertness and reaches its peak around 6am, readying us for waking up; its lowest around midnight getting us ready for sleep

Question 29

What are the evaluation points for circadian rhythms?

Answer 29

✅ practical application into shift work
✅ practical application to drug treatments (pharmocokinetics)
❌ use of case studies and small samples in studies (Aschoff and Wever and Siffre)
❌ poor control in research studies (Czeilsler et al 1999)
❌ individual differences (Duffy eg at, 2001)

Question 30

Discuss how one strength of research into circadian rhythms is its practical application to shift work

Answer 30

Knowledge of circadian rhythms has given researchers a better understanding of the adverse consequences that can occur as a result of their disruption (desynchronisation). shift workers often experience a reduced concentration around 6am (circadian trough) meaning mistakes are more likely. Research has also suggested a relationship between shift work and poor health- shift workers are 3x more likely to develop heart disease. Thus, research into the sleep/wake cycle may have economic implications in terms of how best to manager worker productivity

Question 31

Discuss how one strength of research into circadian rhythms is its practical application to drug treatments

Answer 31

Circadian rhythms coordinate a number of the body’s basic processes such as heart rate, digestion and hormone levels. This in turn, has an effect on pharmocokinetics, that is, the action of drugs on the body and how well they are absorbed and distributed. Research into circadian rhythms has revealed that there are certain peak times during the day or night when drugs are likely to be most effective. This has led to the development of guidelines regarding timing of drug dosing for a whole range of medications, including drugs to treat cancer or epilepsy. Thus, research into circadian rhythms have real-life medical benefits

Question 32

Discuss how one limitation of research into circadian rhythms is the use of case studies and small samples in studies

Answer 32

Studies of the sleep/wake cycle tend to involve small groups of participants, as in the experiment by Aschoff and Wever or, Folkard (who used only 12 participants), or case studies of single individuals, as in the case of Siffre. The individuals involved may not be representative of the wider population and this limits the extent to which meaningful generalisations can be made= reduces population validity of such studies and case studies

Question 33

Discuss how one limitation of research into sleep/wake cycle is poor control in research studies

Answer 33

Although participants in sleep/wake cycle studies were deprived of natural light, often they still had access to artificial light. For example, Siffre used dim artificial light to navigate around the caves and turned on a lamp every time he woke up until he went to bed. The use of dim lighting may be similar to taking a drug that resets their body clock. This suggests that researchers may have ignored an important confounding variable in circadian rhythm research, ultimately reducing the interval validity of the experiments

Question 34

How does blue light ‘reset’ our body clocks

Answer 34

artificial light (blue light) is one of the most significant exogenous zeitgebers, responsible for ‘resetting’ the body clock each day- the SNC contains photoreceptors and receives information regarding light levels from the optic nerve which sets the circadian rhythm so that it is synchronisation with the outside world e.g day and night. If high levels of light (blue light) reach the retina, melatonin production is inhibited and the feeling of drowsiness will not occur.

Question 35

Discuss how a limitation of research into sleep/wake cycles is that individual differences may be influence an influence on results

Answer 35

Another issue which complicates the generalisations of findings from studies of sleep/wake cycle is that individual cycles can vary, in some cases from 13-65 hours. Some people demonstrate a natural preference for going to bed early and rising early (‘larks’) whereas some people prefer to do the opposite (‘owls’). This means that findings from sleep/wake cycle studies may not fully represent individual differences within the population