Biological Rhythms

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  • Created on: 03-06-16 11:39

Biological Rhythms

Biological Rhythms - periodic fluctuations in physiological functioning

The Rhythms

  • Circadian Rhythm (about a day) is a rhythm that occurs every 24 hours e.g. The sleep wake cycle, body tempurature etc.
  • Ultradian Rhythm is a rhythm that occurs more than once within 24 hours e.g. heartbeat, blood pressure, appetite etc.
  • Infradian Rhythm is a rhythm that occurs over a period greater than 24 hours e.g. menstrual cycle etc.
  • Circannual Rhythm occur over a year e.g. hibernation, migration, mating etc.

The role of Endogenous Pacemakers and Exogenous Zeitgebers

Endogenous pacemakers = Internal mechanisms or 'biological clock'

External Zeitgebers = Environmental cues e.g. light/dark, tempurature etc.

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Endogenous Pacemakers

The role of the suprachiasmatic nucleus (Humans)

Main biological clock - In the HYPOTHALAMUS in a group of neurons called the SCN. They have their own regular rhythms of activity. MOST IMPORTANT ENDOGENOUS PACEMAKER. This controls the pineal gland and the release of melatonin via an interconnecting pathway. The pathway connects the retina of the eye to the SCN so that the sight of natural light is conveyed from the retina via the SCN to the hypothalamus.

Isolation studies - all possible external cues relating to time/day are removed from the P.

Research study :- Stephan and Zucker (1972)

Investigated the effects of damage to the SCN on circadian rhythms. Rats housed in the lab with 12 hours of light followed by 12 hours of dark. They drank more and are more active during the dark period. They found damage to the SCN eliminated the normal circadian patterns of drinking and activity.  

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Role of the Pineal Gland (birds and mammals)

  • Less light is detected by receptors in the retina
  • A neurone connects the retina to the SCN
  • A neurone connects the SCN to the pineal gland
  • The pineal gland converts serotonin into hormone melotonin
  • Melotonin is released into general bodily circulation
  • Melotonin makes us sleepy

Binkley (1979) found that chickens wake and become active as dawn breaks and the melotonin levels fall. This suggests that although the waking cycle is controlled by the pineal gland, it is adjusted to the actual time that morning begins, which varies throughout the year.

Body tempurature

The rhythmic variation in BT is an example of a circadian rhythm. It's at its lowest at 4:30am and highest around 6pm. It therefore appears that our sleep-wake cycle and body tempurature are closely linked. BT is a circadian not an ultradian as the highest and lowest BT points occur once a day.

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Research Study: Michel Siffre (1975)

Michel Siffre spent 6 months in an underground cave in Texas. There was no natural light, and in the absence of this critical zeitgeber biological rhythms became what is called FREE RUNNING. 

There were two key findings:

  • His sleep-wake cycle extended from 24 hours to 25-32 hours. When he emerged on the 179th day he estimated he had been in there for 151 days.
  • His body temp circadian rhythm was more stable. It extended slightly to 25 hours but remained consistent. One outcome of this was that Siffre's sleep-wake cycle became desynchronised from his body tempurature rhythm. Under normal conditions they are synchronised so that we regularly go to sleep when body temp is falling and awake when its rising.

In the absence of environmental zeitgebers such as light our endogenous pacemakers can still regulate biological rhythms. However, this regulation is not perfect so the rhythms are not perfectly tuned into the 24hr cycle.

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Evaluation of Michel Siffre and Biological approac

Individual differences - These kind of studies are very rare and usually only involve one or small number of P's and so are called case studies. The results may not apply to the general population. Circadian rhythms can vary dramatically in some people and we know that their morning people and evening people. In Michel Siffre's case, his bodily behaviour may not be typical of all people.

Approaches - The findings from this study can be explained from a biological perspective. This explains circadian rhythms in terms of physiology such as brain structure, hormones and genes. It is not always appropriate to explain biological rhythms in these terms. Human functioning is much more complex than this and biological rhythms can be overriden by choice.

Biological explanations are an appropriate level for the study of sleep as sleep is clearly a behaviour driven by our biology. However, biological explanations are reductionist, and this has its advantages. For example, if we study just melotonin levels and eliminate other factors we can establish cause and effect.

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The role of EP and EZ on the menstrual cycle

The menstrual cycle is an example of a human biological infradian rhythm, Research studies indicate that the menstrual cycle is mainly regulated by endogenous pacemakers but that exogenous zeitgebers also play a part. 

One zeitgeber that helps regulate infradian rhythms is the release of pheromones from other people.

Russell et al (1980) took a daily sample of sweat from one group of women and rubbed onto the upper lip of a second group of women. The two groups of women were kept seperate for entire study but their menstrual cycles became synchronised with their 'sweat donors' cycle.

The results of this and other studies have been heavily critisised by a number of researchers on the grounds of methodological flaws. Some researchers refute the claim that pheromones affect the menstrual cycle in any way. 

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Consequences of disrupting biological rhythms

Jet Lag - the physiological effects of disrupted circadian rhythms

Jet lag can lead to dislocation between body clock and local zeitgebers - an unpleasant feeling of extreme tiredness and slowed mental and physical reactions. The effects of constand travel can  be:

  • Headaches
  • Loss of appetite
  • Nausea
  • Shortness of breath
  • Sweating

Cho (2001) investigated the long-term effects of frequent jetlag. They found that Ps who were allowed only a few days rest between flights were more jetlagged compared with those allowed longer recovery periods. Repeated jetlag with insufficent recovery time can lead to reduced cognitive ability and brain damage. 

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The cause of jetlag

The problem of jet lag is caused by our endogenous pacemakers being out of phase with exogenous zeitgebers. Travellers experience a desychronisation between internal biological cues and external environmental cues.

Most people find the effects of jet lag are reduced when flying from east to west than vice versa. This is because they are following the sun and therefore effectively extending the length of their day.

Recht et al (1995) examined the outcome of baseball games in America which is a country large enough to have 4 time zones. They found that when teams played away games they were more likely to win if they travelled east to west than vice versa.

Various factors affect the severity of jetlag:

  • Direction of travel
  • Number of timezones crossed
  • Age - decreases with age
  • Individual differences - some people are resistant to the effects of jet lag, phase tolerance
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Overcoming jetlag

There are a number of techniques that have been shown to reduce jet lag according to Cohen (1996):-

  • Attempt to adjust to the time zone of the local destination as soon as possible in order to reset endogenous pacemakers.
  • Sleep well before the flight, so you don't start sleep deprived.
  • Avoid caffeine and alcohol as they make symptoms worse
  • Go out into the morning daylight as soon as possible - sunlight is the most effective way of resynchronising your body clock to local time
  • Melatonin has been studied as a possible treatment for jet lag. Takahashi et al (2002) found that melatonin speeds up the resynchronisation of biological rhythms after an 11 hour flight reduced the effects of jetlag.

Amir and Stewart (1996) suggested that light may not be the most important zeitgeber. They have shown that rats who recieve a breeze before their wakeful period can reset their clocks by a chance in time of the breeze alone. There may be contingent signals which help to maintain our circadian rhythm which are absent when we travel abroad, for example, the usual time we eat, regular TV. 

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Shift Work

Working shifts means that physiological rhythms are out of phase with each other and the enviornment. Some people can take up to three weeks to adjust to a single week of shifts.

Internal desynchronisation - describes the imbalance of these rhythms.

People who work shifts are more likely to drink more alcohol, take more tranquilisers and sleeping pills, and report more job stress and emotional problems. The best shift pattern is to do a week of earlies, then lates, so the they are constantly extending their days.

Costa (1999) - Research found that long term effects of shift work include:

  • Difficulties in social and family relationships
  • Development of peptic ulcers
  • Chronic fatigue anxiety and depression
  • Cardiovascular problems
  • Pregnancy difficulties
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Research studies for shift work

Czeisler et al (1982)study of a Utah chemical plant provides experimental support. Czeisler persuaded a company to change their shift patterns to a phase-delay system and increase shift rotation to every 21 days. He found worker satisfaction increased and factory output was higher.

Sharkey (2001) found that the hormone could speed up the biological adjustment to shift patterns and increased sleep times during non-work periods.

Coren (1996) estimated that on average we sleep for around one and a half hours less than we did a century ago, so that many of us are in a constant state of sleep deprivation.


Research studies on shift work offer suggestions about how the negative consequences of shift work can be reduced. Czeisler suggested that a phase delay system would be beneficial. 'Nap breaks' during shifts have been shown to reduce tiredness and improve performance. 

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Evaluation of disrupting biological rhythms

Research findings: A strength of this research is the consistent evidence that disrupting biological rhythms can have cognitive and emotional effects on people. People who have lives where biological rhythms are disrupted for long periods suffer significantly more health problems. This research allows us to find solutions and recommendations for how people can make changes to reduce the impact of disruption.

Methodological issues: Much of the research is on field studies which are high in eco validity but are not well controlled.There are many individual differences between P's making it difficult to draw conclusions.

Real-life significance: Many people do shift work for long periods of time with no real consequence to their health. Western societies rely on shift work and without shift workers the NHS, fire brigade and police would not be able to offer their services at night. 

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