Biological Rhythms

  • Created by: ava.scott
  • Created on: 10-03-15 10:52

Cicardian Rhythms

  • Controlled by endogenous pacemakers
  • Influenced by exogenous zeitgebers

The main pace maker is the suprachiasmatic nucleus (SCN).

  • Small group of cells in hypothalamus
  • Just above the optic chiasm and connects the retina of the eye to the SCN.
  • This allows light to influence the activity of the SCN.
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SCN mechanism: Name and 6 points

How does the SCN work?

Kalat hypothesised the SCN generates its own rhythm by protein production.

  • The cells in SCN produce a protein for a period of hours until the level inhibits further production, again for hours.
  • When the protein concentration drops below a threshold, the protein is produced again.
  • The biological rhythm activates the pineal gland (connected to the SCN) causing it to release melatonin, which causes drowsiness.
  • Melatonin is released during the night, in the dark.
  • Light prevents the protein production of the SCN, and so melatonin levels fall.
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Exogenous Zeitgebers

Zeitgebers help synchronise the sleep-wake cycle.

  • Examples are light and dark.
  • Light is the dominant time cue for cicardian rhythms.
  • It does this through light sensitive cells in the retina , which tell the brain whether it is nightime or day time and adjusts our sleep patterns accordingly.
  • When new light patterns are used to adjust the sleep wake cycle (e.g. when travelling across time zones) this is called entrainment.
  • The sleep-wake cycle exists without light, but it does influence the cycle when present.
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IDA: for sleep wake cycle G3 B1

Biological approach:


  • Easy to test empirically
  • lots of practical applications e.g. melatonin pills
  • Understands mechanisms clearly
  • This is positive for the theory as we can prove the causality behind drowsiness and alertness in an laboratory, and therefore justifies the theory. Practical applications increase the worth of the theory as it has many uses and can help people.


  • Reductionist: Does not explain cultural variation in sleep patterns. The spanish population, and many hot meditterarean countries, have a siesta at the brightest part of the day. this goes again light being a suppressor of melatonin. There must be either social factors or other factors affecting the sleep wake cycle e.g. temperature.
  • This is negativee for the thoery as it doesn;' explain a certain aspect of sleep in humans, and therefore cannot be consider a comprehensive explanation of biological rhythms.
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Research evidence: Morgan + Eval

Removed SCN's from hamsters and found their cicardian rhythms disappeared.

Also transplanted mutated SCN's into hamsters and found that the recieving hamster took on the mutant cicardian rhythms.


This supports the biological explanation and the role of the SCN in the sleep-wake cycle, as its removal and transplant had a direct effect of the sleeping patterns and behaviour of the hamsters. Therefore, the SCN must work as a endogenous pacemakeker, and is less effected by light and dark.


Good: Empircial and scientific method. Highly controlled.

Bad: Hamsters aren't humans so cannot generalise. The artifical settings make it less generalisable to hamsters in the wild, let alone humans. Hamsters don;t have the same social influences as humans, so we may be less effected by the SCN's protein production.

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Research evidence: Shochat

Studied the role of Melatonin in sleep.

  • 6 male participants
  • spent 29 consecutive hours ina  sleep laboratory
  • They had to spend 7 minutes of every 20 minutes lying down in a dark room trying to sleep.
  • The period people find it easiest to go to sleep in is known as the sleep gate.
  • Blood samples were taken every 20 minutes to allow the measurement of melatonin.
  • The release of melatonin preceeded the sleep gate by 100-120 minutes.
  • Also found those with insomnia found it easier to sleep when given a melatonin pill two hours before.


Supports the theory as Melatonin production was released from the pineal gland, on average, before drowsiness came about. This shows that it may play a role in the sleep gate. Those with insomnia also improved when given melatonin, so this is also  suggests a link between the chemical and sleep.

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Research: Evaluation of Shochat g3


  • empirical methods e.g. using frequent blood tests to analyse levels of chemcials in the blood.
  • High controlled situation leads to less extraneous variables.
  • The research is repeatable and therefore will give reliable results. This is good for the theory, and the results appear to support the role of melatonin.


  • Very low populational validity- only 6 male particpants.
  • Artifical settings- may have suppressed/encouraged drowsiness differently to real life.
  • The darkness may or may not have stimulated the release of melatonin, and affected how much and when it was released.
  • This makes the research less applicable to real life situations, and therefore we cannot know if the sleep gate is actually influenced as directly by melatonin as in this artificial setting. We also can't assume that everyone's sleep gates are the same as these 6 participants.
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Research Evidence: case studies


Michel spent 2 months living in a dark cave. He developed a 25 hour sleep-wake cycle, when the average one is 24 hours.


Supports the role of light as a exogenous zeitgeber in the cicardian rhythm, as without it a different wake cycle develops. However, we dont know if 25 is the 'natural' endogenous rhytym, or is just developed due to abnormal circumstances. Diet and other factors may have also effected Michel's rhythm, not just the absense of light. Low pop validity.


Miles wrote about a man who had no sight from birth and developed a 24.9 hour sleep-wake cycle, despite exposure to clocks, alarms and radios. He had to use sedatives to help him sleep at night, and stimulants during the day.

Also supports the role of exogenous zeitgeber light, as the absense of it resulted in a different cicardian rhythm. However, low pop validity. Blindness effects many different lifestyle factors.

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Research evidence: Mariana Figeuiro, ipads and eva

They wanted to find out the effect of self luminated tablets had on melatonin suppression.

  • They observed and gathered data on 13 volunteers watching films, playing games amd reading documents.
  • They found that two hour exposure to self-luminous light from a screen can suppress melatonin by about 22%. Stimulating the cicardian rhythm may effect sleep when used prior to bedtime.


This also supports the role of exogenous zeitgebers as the retina will be processing the light from teh tablets as daytime, and therefore will not stimulate the SCN to release melatonin. This could result in entrainment,as the light acts as a cue for the cicardian rhythm.

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Mariania Figeurio evaluation


  • High control over variables.
  • Observational studies are easy for bias to occur.
  • Low populational validity- only 13 participants.
  • Artificiality may result in lack of generalisability.
  • Practical applications- reduce use of ipads before bed.
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Research evidence: Overall evaluation


  • Many repeatable studies, so reliability is high.


  • Low populational validity in all studes- especially case studies. This means we cannot use the results to generalise for the whole population. (MICHEL, HAMSTERS,SHOCHAT(6men)
  • Artifical studies in the laboratories may have resulted in abnormal melatonin/sleep-wake cycles.
  • Doesn't look at other factors such as temperature/social stimulation.
  • Low control in case studies.
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Wider Evaluation

  • Many practical applications to help improve sleep- reducing light around you, reducing luminous tablet use, melatonin tablets.
  • Use of animals in Morgans study is unethical
  • Difficult to study as depriving someone of sleep/ light is unethical.
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Disruption: Jet Lag

Jet lag is the extreme tiredness experienced after long haul flights over different time zones.

  • It results in the temporary misalignment of the endogenous pacemaker, time zones and sleep wake cycle.
  • SYMPTOMS: Sleep disturbance, cognitive effects, fatigue, irritability, headaches, digestive problems.


  • Phase delay is when the cicardian rhythm is lengthened by the need to go to sleep later and wake up later.
  • Phase advance is when the cicardian rythm is shortedned by the need to go to bed earlier and wake up earlier.
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social jetlag


  • The discrepancy between your natural body clock and your social clock- the schedule you keep for your job and other commitments.
  • You body is essentially going back and forth through timezones each day and week, resulting in sleep-deprivation.
  • Same symptoms.
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Research evidence for jet lag: Klein


  • Confirmed that flying west to east causes more jetlag than east to west.
  • They did this by looking at flights from the UK to the US. and taking the passengers temperature-rhythm one day before they left, and then the 8 days after they arrived. 
  • Those who flew from US to UK could entrain more easily than those flying UK to US.
  • One day per time zone crossed was needed for recovery.


This supports the idea that adjusting to a slightly longer cycle is easier than a slightly shorter one, as without zeitgebers (as in case studies) our natural bodyclock runs a 25 hour cycle. West to east is phase delay (lengthens rhythm) because the east is behind the west in time differences. East to west is phase advance, which we find more difficult.


Loads of different factors: Temperature of the place they have arrived may make a difference, food differences. Otherwise repeatable and reliable methodology.

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Research evidence for Jetlag: Sasaki

Sasaki (1980)

  • Reported on the performance of the soviet union volleyball team after travelling the plan in Japan. The team lost matches the first three days, but won by increasing margins over the next six.
  • Sasaki said this was due to the re-entrainment of the soviet team to the Japanese time zone.


Supports the theory as travelling east to west is difficult resulting in less wins. However entrainment lead to a increase in wins, as the players body cycles readjust.

Evaluation: There are other factors rather than just the cycle. New food, temperature, air quality, or just a new environment may have caused the fewer wins.

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Research evidence for jet lag: Schwartz


Found that american football teams travelling east to west had significantly more wins than those travelling west to east.


Supports the theory as the teams travelling west to east would have to phase advance by three hours, which is more difficult than phase delay. Therefore their circardian rhytm would be more out of sync with exogenous zeitgebers than the EAST coast teams.


Low face validity- perhaps west coast teams are just worse at football.

Different factors may effect.

Low population validity- only looks at football players and the effects of jet lag on their performance in a game.

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Research evidence for jet lag: O'Conner

Looked at the ffects of tracelling across time zones for competitive swimmers.

Found no significant evidnece of a drop in performance that couldn't be explained by a lack of training due to evidence.


This doesn't support the theory, as it says that jet lag has no effect on performance of swimming.

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Research evidence jetlag and cancer/cortisol

Rafnsson found that out of 1500 flight attendants, those who has been flying for over 5 years had double the risk of breast cancer.

Grounding: Good for the theory as it suggests that sleep disruption could reduce restoration, and therefore cause ill effects ont he body. HOWEVER MAY FACTORS: high altitudes, greater exposure to UV, stress levels and boredom.

Kojo found that there was no increase in breast cancer among cabin crew that couldnt been attrtibuted to risk factors.


Inconsistent evidence.

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Reserach for jet lag overall for performance

Evidence seems to support overall that jet lag negatively effects performance, especially when flying west to east.

However, the evidence is not hugely populationally valid. Sports studies aren't too scientific and could be down to many factors.

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Disruption: Shift work

Lorng term disruption which includes working irregular patterns e.g. night shifts.

Jobs include cleaners, carers and doctors

Disrupts the sleep wake cycle becayse of changing patterns each week menas no adjustment can take place. Night shifts are idffiuclt as exogenous zeitgebers such a light and social factors keep you alert wehn you need to be sleeping and vice versa for when you need to be working.

Certain affects of shift work:

  • increased Cardio vascular disease
  • mood swings
  • risk of accidents at work
  • family problems
  • increased risk of cancer
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Disruption: IDA shift work

Socially Sensitive

Shift work is often found in lower paid jobs, and so people are risking these effects for less money. This is sensitive to those working.

Also, mistakes made do not not want to be blamed on employee's tiredness.

Those being cared for and recieiving a service at night time don;t want to feel at risk because of shift workers of duty, but often shift work is unavoidable for certain jobs.

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Research evidence Shift work: Blakemore

Studied workers in a chemical plant in Utah who worked 3 weekly schedules. The first week was a night shift, then an evening shift and then a day shift.

Animals subjected to this shedule suffered heart disease and a shorter life span.


Shows how phase advance is very difficult and damaging. The workers are having to shorten their cicardian rhythm, as the weeks go on, and then suddenly jump to a night shift again. Animals are very damaged by this.


Chemical shift workers in Utah-- SMALL POPULATIONAL VALIDITY

Animals- ungeneralisable and unethical!

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Research evidence Shift work: Czeisler

Proposed it would be better for shifts to become later as the weeks go on, so begnning with a day shift and ending with a night shift. He tested this in the same chemical plant as Blakemore. 

Workers reported feeling better an dless tired on the job. Managers reported less accidents and increased productivity.


This is an example of phase delay (lengthening circardian rhythm), which is easier to adjust too, as our natural circardian rhythm without exogenous zeitgebers is actually longer anyway. Therefore, this new rota is easier to adjust too throughout the week. 


Workers are still jet lagged, although not as badly. Practical application does not solve all problems.

Placebo effect- any change to routine may seem an improvement at first, until it becomes the norm again.

Small population validity still- maybe just these workers prefer phase delay?

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Research evidence Shift work: Breast Cancer?

Australian study by Fritschi

  • 1000 women
  • showed no link between shift work and breast cancer

Danish Military by Hansen

  • 692 women
  • found a link
  • working night shifts associated with  40% rise in breast cancer
  • shift workers who considered themselves morning people had 4 time shigher risk than women who work during the day.

Melatonin can effect other hormones taht incluence breast cell growth. Breast cancer patients have lowe melatonin levels anyway. Shift work suppresses it further using light and noise and stimulation. Morning people also find it more difficult to adjustto hormonal and metabolic chnages from shift work, and so this increases stress and then increasing risk of cancer.

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