Circadian Rhythms.

This is just to help me revise for my january's :) but anyone can use.

Circadian Rhythms.

Circadian Rhythms are rhythms which last around about 24 hours in length.

The two most prominent types of circadian rhythms are;

1. The sleep wake cycle.

2. Core body temperature.

All organisms are exposed to fluctuations in light and dark in an altering 24 hour rhythm.

The sleep wake cycle is a very obvious cycle as all animals sleep at least once in a day, even if they differ in length.

Core body temperature is less obvious still affects us every day. Our core body temperature is at its lowest at about 4.30 am (36 degrees) and highest at 6.00pm (38 degrees.)

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Sleep Wake studies research.

Siffre (1972)

  • Spent 205 days in a midnight cave in Texas.
  • He could illuminate his surroundings the way that he wanted, however, he had no way of knowing the time in the outside world.
  • Researches found that at first Siffre's sleep pattern was erratic but it soon settled into a free running rhythm.
  • They also found his day shifted to 25 hours in length.

This study is supported by Aschoff (1976)

  • Had pp's spend up to a month living in specially designed bunkers, which were build to shield pp's from any external cues.
  • It was noted that the participants kept a regular cycle of activity but after a few weeks pp's were found to be half a day out of synch with the rest of the world.
  • They found that the participants sleep wake cycle is maintained but it became free running.
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Lavie (2000)

  • Argued that the aritifical lighting in these studies artificially lengthened the pp's circadian rhythms.
  • He said that the actual free running rythm is closer to 24.2 hours.

Richter (1968)

  • Recorded the rest activity cycles of a blinded squirrel monkey for over three years.
  • Whilst its cycles became free running the rest activity cycles themselves varied by only a few minutes during this time.

However, the Richter study is conducted with animals therefore it cannot be generalised to the human population.

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The importance of the blind...


The vital importance of light as an exogenous zeigeber in the circadian rhythms of totally blind individuals. Consequently many rhythms become desynchronised, such as the sleep wake cycle, temperature and hormonal rhyhms.


Skene et al (1999)

  • Said that the majority of totally blind people suffer with non-24 hour sleep-wake disorder.
  • This is characterised by disrupted sleep cycles and excessive day naps.
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The SCN.

Stephan and Zucker (1972)

  • Discovered a specific region of the hypothalamus as the site of the circadian clock.
  • They found that lesions to a group of cells called the superchiasmatic nucleus (SCN) affected a number of circadian rhythms in rats, including hormone secretion and drinking and wheel running.

Inoyue and Kawamura (1979)

  • Kept animal SCN tissue alive in an organ culture so that it was isolated from all other brain activity.
  • They found that even in the absence of light information from the eyes, the SCN tissue continued to exhibit circadian rhythms.
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Klerman et al (2002)

  • Said that the most reliable circadian rhythm is dim light melatonin onset (DLMO)
  • The hormone melatonin is released by the pinal gland and suppressed by light.
  • Concentrations begin the rise in the evening with dimming light.
  • Levels of melatonin are constant throughout the night (as long as it remains dark) and then decline towards the time of natural waking.
  • The morning light suppresses the melatonin and the cycle starts over.

Boivin and James (2002)

  •  found in there research that even normal room light can supress melatonin.
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Is the SCN just in a persons eyes?

There is substantial evidence showing that a wide range of species have specialised non-visual photreceptors.

Foster et al (1991)

  • found that mice bred to be blind and non-responsive to light can be entrained to a light dark cycle.

Lui et al (1997)

  • Removed the head of a fruit fly and found that certain circadian continued in the body for up to 3 days
  • This suggested that the head and brain did not contain the sole circadian clocks.


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Yang et al (1998)

  • Found that they could influence the circadian rhythm of fruit flies that had been selectivley bred to have no eyes.

Weber (1995)

  • found that cells sensitive to light within the skin of cockroad legs serve as a circadian clock.


1. Because the research is done with animals it cannot be generalised to the human population.

2. This research could be argued as unethical as is caused the animals in the studies a lot of pain and ultimatley death.

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Some Extra Research (SCN)

Ralph et al (1990)

  • They took the SCN from a golden hamster which had a mutated circadian rhythm of 20.2 hours and transplanted it into hamster with a normal circadian rhythm.
  • They found that on recovery, the recipient hamster, exhibited a circadian rhythm very similar to that of the donor.
  • This circadian rhyth could only be attributed to the transplanted SCN.

Further research also suggested that individual cells in the SCn could sustain circadian rhythms.

Welsh et al (1995)

  • found that there was substantial variation maintained by individual cells, ranging from 21.25 - 26.25 hours.


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