Biological Rhythms and Sleep

  • Created by: Amy
  • Created on: 04-01-13 15:04

Biological rhythms

There are three main types of body rhythms:

  • Circadian rhythms: approx daily e.g body temp, hormones such as prolactin (producing milk in women), sleep/wake cycle
  • Infradian rhythms: long, slow rhythms, e.g hibernation, migration, menstruation, SAD
  • Ultradian rhythms: short, faster cycling rhythms e.g deep and light sleep cycle

These rhythms have evolved through natural selection to allow animals to predict and to behave appropriately in the natural world.

Biological rhythms need two things to work effectively:

  • Endogenous pacemakers (body clock)– internal cues; keeps time and keeps bio rhythm following a regular pattern.
  • Exogenous zeitgebers (time givers) - external cues, eg the sun
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Circadian rhythms are approximately daily. These include the body temperature cycle, which peaks in the afternoon and has a low point in the early morning. There are many other daily rhythms, especially in the release of different hormones. The most obvious circadian rhythm is the sleep/wake cycle. This occurs approximately once every 24 hours.

Circadian rhythms, like other biological rhythms, are controlled by a body clock, or endogenous pacemaker which is located in the suprachiasmatic nucleus or SCN.

The SCN is located in the hypothalamus and inks with light receptors in the eye. It is linked to the pineal gland and is responsible for release of melatonin. The sleep/wake cycle is reset when the exogenous zeitgeber of light reaches our eyes and causes us to wake.

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Silver - Study to show location and function of pa

Which part of the brain was removed?


What was the effect on the hamsters’ circadian rhythms?

Irregular activity – they lost their daily pattern

What does this show?

The SCN controls your circadian rhythms

This was a well controlled study, although it does not prove on its own that the SCN is the body-clock. By removing the SCN the researchers must have also damaged other areas nearby and it could have been this that damaged the activity cycle. Therefore Silver carried out a second experiment:

What was implanted in the hamsters’ brains?

Fetal tissue of the SCN

What was the effect on the hamsters?

After a few days, they regained normal circadian rhythms

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Evaluation of Silver


  • There seems to be a causal relationship between SCN and bio rhythms therefore study supports endogenous pacemakers role in bio rhythms


  • Animals: difficult to generalise the SCN in humans may not play the same role in human bio rhythms
  • Not ethical study as harms animals
  • Damage to other areas of brain may have caused the change
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Siffre (1975)

Where did he carry out his study?

A cave in Texas

How long for? 

7 months

What did he measure, and how?

When he slept and woke, blood pressure, body temperature, ability to do manual tasks (vital functions)

What were his main findings?

his daily rhythms occurred on a 25-hour clock

Evidence for an endogenous pacemaker –

He still had a cycle even without exogenous zeitgebers

Evidence an exogenous zeitgeber is needed –

We have 25-hour clock, so we need exogenous zeitgeber to keep our clock regular

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Evaluation of Siffre


  • High internal validity; uses scientific measures and removed all exogenous zeitgebers therefore support for the theory
  • Psychology as a science, objective measuring to monitor bio rhythms therefore more reliable approach


  • Difficult to generalise because there was a restricted sample, unique case therefore may not apply to biological rhythms in everyday situations
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Infradian rhythms include annual rhythms which prepare animals to behave adaptively in different seasons eg: preparing to breed in early spring; to migrate in early autumn and spring; to hibernate in late autumn. Also lunar rhythms are approximately 1 month long and allow marine animals to synchronise their breeding cycles with the tides.


Pengelly and Asmundson carried out a study on Golden-mantled ground-squirrels, which hibernate in the wild.  Five captive-born ground-squirrels were blinded after birth.  They spent their lives in constant darkness and constant temperature with plenty of food.  For the next four years the ground-squirrels hibernated at roughly the same time as wild animals.  This is a clear demonstration of an endogenous bodily rhythm.  Their body-clocks were not completely accurate as some of them hibernated a bit earlier each year.  

Evidence for endogenous pacemaker:

They still hibernated even without exogenous zeitgebers

Evidence for exogenous zeitgeber: 

Their body clocks were not completely accurate as wild animals

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  • High internal validity (controlled) provides strong support


  • Can't generalise to humans (may work differently with squirrels and we don’t hibernate
  • Ethical issues - cruel 
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Examples of infradian rhythms in humans

1. Seasonal Affective Disorder (SAD): what causes it and how can it be treated?

A form of depression that in some people repeatedly occurs during the winter months. It may therefore be associated with abnormalities of biological rhythms. Evidence to support this is that light therapy or treatment with melatonin can be helpful

This suggests exogenous zeitgebers exist because they’re affected by the lack of light and fixed by light therapy.

2. The Menstrual Cycle - What happens to the menstrual cycle when women live together for long periods?  Russell et al (1980) study: Found that if sweat from one woman (donor) was rubbed on to the lips of a recipient woman, eventually the menstrual cycles of donor and recipient would synchronise. Their explanation was that phereomones in the sweat were picked up by the recipient and these altered the menstrual cycle of the recipient. Evidence for endogenous pacemaker: They would still have their period if they didn’t synchronise Evidence for exogenous zeitgeber:  The sweat synchronised their periods

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In evolutionary psychology, why would it be advantageous for women living together to have synchronised menstrual cycles?

All women would be fertile at the same time

Issues and Debates:

  • not reductionist,as explains endogenous and exogeonous pacemaker and zeitgebers
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These are rhythms of less than a day. The main ultradian rhythm comes from the pattern of human sleep when we move systematically through phases of sleep - from light slow wave sleep to deep slow wave sleep, back up to light slow wave sleep, then into a period of rapid eye movement (REM) sleep. This occurs in a 90 min cycle, repeated approximately 5/6 times a night.

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Many features of modern life can cause disruption to people’s circadian rhythms. Electric light makes it possible to extend our day, although this means that people tend to become sleep deprived. Two factors cause considerable disruption: shift work and jet lag.

Shift Work and the consequences of disrupting circadian rhythms 

 Shift work disrupts circadian rhythms by making people work when they would normally be asleep, interrupting the normal cycle of sleep and wakefulness. Changes in shifts also force people’s bodies and minds to adapt to a new pattern. However, different biological rhythms like those in hormone release, body temperature, digestion and alertness, all adapt at different speeds –so the person can be in a state of internal desychronisation for several days.

Why is itimportant to consider the impact of shift work?

20% of people work shifts (USA), Because of people being tired, the reactor in Chernobyl exploded, more lorry accidents between 4-7am

Internal desynchronisation where each cycle in the body takes a different amount of time to adapt to the new exogenous zeitgebers


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Czeisler (1982)

Where was it carried out? 

A quarry

Who were the participants?

Shift workers

What was the original shift pattern?

Night, Day then Evening shifts

What were the effects of this disruption of the participants’ circadian rhythms?

They were tired and the boss was concerned about their performance

What was the shift pattern changed to?

Day, Evening and Night shifts 3 weeks at a time in that order

What were the results?

If you’re working with the biological clock, it is much easier and causes less disruption (phase delay)

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  • High external validity
  • reflects real life therefore we can use this study to affect shift work in real world settings


  • Carried out in America therefore culture bias, so harder to use to explain the effects of shift work
  • Low population validity restricted to one profession therefore cannot apply to all professions

The implications of Czeisler’s work are that people can be helped to adjust to shift work if we effectively use our knowledge about our natural circadian rhythm and the most effective exogenous  zeitgebers (light). However, there is debate about whether rapidly rotating or slowly rotating shifts are better. 

Coren claimed rotating shift patterns every 2 or 3 days avoids even trying to adjust the circadian rhythm. However it means your body is permanently in a state of desynchronisation.

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Additional research into shift work.

Gordon et al (1986) study:

Police officers – moved from backward to forward shifts (day, evening, night instead of night, evening day,) lead to 30% reduction of sleeping on the job and 40% reduction in accidents at work


Phillips introduced permanent shifts in Kentucky Police, workers would choose from one of three 8 hour shifts because their body isn’t having to repeatedly adjust to a new pattern

Sack Sack provided places for planned napping. But it wasn’t popular with the staff

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Jet Lag and the consequence of disrupting circadia

This is caused by rapid travel across time zones by plane - too fast for the body to adjust the endogenous pacemaker in the SCN.


Extreme tiredness, slowness of mental and physical reactions

Why does jet lag occur?

Your endogenous pacemakers are out of sync of the exogenous zeitgebers.

Explain why there is less disruption when flying E> W

Because our body runs on a 25 hour clock and it’s easier to stay up a few more hours rather than go to bed when we’re not tired

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Supporting study: Schwartz (1995)

Who did they study?

US Baseball teams; some were travelling west to east, some travelling east to west

Which teams had the best performance? 

The ones who were travelling east to west; 44% games won

Which had the worst performance?

 West to East 37% games won

Explain why

Easier to just to phase delay rather than phase advance


Difficult to establish cause and effect - is it direction of travel or sporting ability therefore this study may not tell us about the effects of direction of travel

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Four techniques that have been shown to reduce jet lag (Coren 1996)

  • Sleep well before the flight – so don’t start off sleep deprived 
  • Anticipate the time on arrival e.g. arriving in daylight, stay awake on the plane
  • Avoid caffeine and alcohol which can worsen jet lag symptoms
  • Adjust to local zeitgebers on arrival
  • Go out into daylight as soon as possible as sunlight resynchronises your body clock to local time
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