Biological Rhythms

Circadian Rhythms

AO1 = Circadian Rhythms are those that last about 24 hours, the two best known circadian rhythms are the sleep wake cycle and the body temperature cycle most if not all our bodily functions exhibit a 24 hour cycle.

These cycles are thought to be controlled by;

  • Internal biological processes ( endogenous pacemakers or body clocks) ........ which are readjusted every day by..........
  • External environmental cues ( exogenous zeitgebers or timegivers)

The internal circadian clock is free running, it works without external cues, setting a cycle of about 24-25 hours. However under normal circumstances the internal clock does not work alone. There are external cues, most importantly daylight, and these help adjust the internal clock to the enviornment in which you live in.

There are research studies which provides evidence that circadian rhythms persist despite isolation from natural light, which demonstartes the existence of an endogenous 'clock', However, research also shows that external cues are important because the clock was not perfectly accurate: it varied from day to day.

1 of 18

Circadian Rhythms

AO2 = support - Michel Siffre

Spent 205 days in a cave underground in 1972. He could illuminate his living quarters when he wanted to - but had no way of knowing what the time was in the outside world, there was an absence of envioronmental cues.

Results = His sleep patteren was erratic at first, but soon settled. His behaviour and his phsiology mainatined a cyclical pattern. His day increased to 25 hours rather than 24 hours long. This shows the existence of a strong free running rhythm. 

This suggests that = An internal biological clock was working and secondly our biological clocks, need external zeitgebers to keep a 24 hour pattern.External zeitgebers are external stimuli which set our biological clocks, examples are light temperature and social cues such as time eat and sleep.They normally change slowly.

2 of 18

Circadian Rhythms

AO2 = support - Michel Siffre 


  • A strength of this study is that it had good control over external zeitgebers which makes the results more reliable, as it rules out any extraneous variables. 
  • A further strength is that the study, is that it was conducted over two months which is a long period of time, which enhances the reliability and validity of the results.
  • A final strength of this study is that this is case study, which is in depth with rich and detailed information. 
  • However, although this case study provides support for the role of endogenous pace makers because it was only conducted on one person, a limitation of this study is that the findings may be difficult to generalise to the whole population. Therefore this study lacks population validity as the finding may not be applicable to everyone Siffre could be unique, this study must be generalised with caution.
3 of 18

Circadian Rhythms

AO2 = Support - Aschoff and Wever 1976

  • Placed participants in an underground WWII bunker, in the absence of envioronmental time cues. 
  • They found that most people displayed circadian rhythms between 24 and 25 hours, although some rythms were as long as 29 hours.
  • This study shows evidence for the existence of a free running rhythm working, in the absence of external stimuli. However, it is important to highlight that some rhythms were as long as 29, which suggests that external cues are still an important controlling mechanism incontrol of circadian rhythms.


  • A limitation of this study is that socail cues should be taken into consideration, as the participants may have influenced one another which makes the findings inconclusive. 
4 of 18

Circadian Rhythms

AO2 = Support - Folkard et al 1985

  • 12 participants lived in a 'temporal isolation unit' isolated from natural light and other time cues.
  • They agreed to go to bed when a clock said 11:45pm and get up when it said 7:45am.
  • The clock ran normally but gradually quickened until it was indicatingthe passing of 24 hours in only 22 hours.

Only one participant was able to keep pace with the 22 hour regime. The others could not. This shows that most of us have a strong free running rhythm.

However, a limitation of this study is that it does not take in to consideration individual differences which may have had an effect on the results making the results less reliable.

5 of 18

Circadian Rhythms


  • It is likely that our actual free-running rhythm is closer to 24.2 hours - Lavie, 2000. For this reason, it needs daily assistance for external cues to entrain (re-set) it to the natural environment.
  • It appears that even in free-running conditions when cycles are not bound to the 24 hour light-dark cycle - endogenous clocks are extremely accurate.
  • But light has been shown as a vital enviornmental cue, this shown by blind people, for whom light does not reach the circadian clock in the brain, have deynchronised rhythms. The majority of blind people suffer from disrupted sleep-cycles and excessive day naps. Blind can not detect light, but they do rely on other cues such as social times to eat and get up.
6 of 18

Circadian Rhythms

Evaluation of sleep-wake research

  • Early research was flawed. Participants were isolated from variables such as clocks, radios and the daylight that might affect circadian rhythms but they were not isolated from artifical light because it was thought that dim light, in contrast ro daylight, would not affect the circadian rhythm.
  • Recent research shows that this may not be true, Czeisler et al 1999 altered participants circadian rhythms down to 22 hours and up to 28 hours using dim lighting.

Individual differeneces

  • Cycle length- circadian cycles can vary anywhere from 13-65 hours.
  • Cycle onset- there are individual differences in terms of when circadian rhythms reach their peak.
  • Morning people rise at 6 am and sleep at 10pm
  • Evening people rise at 10am and sleep at 1am
7 of 18

Circadian Rhythms

Core Body temperature

Core body temperature is one of the best indicators of the circadian rhythms. It is lowest at about 4.30 am about 36C and highest at around 6.00pm about 38C. There is a slight trough just after lunch - the dip occurs even when people have not eaten. In many countries the pratice of having an afternoon siesta is related to this dip in body temperature. The temperature dip is a bi-daily rhythm.

Evaluation of temperature research

We know about at least two biological clocks-

  • Circadian temperature rhythm
  • Circadian sleep-wake cycle

It has been found that nurses on shift duty were able to adjust their sleep-wake cycles quite quickly to their shift pattern, but their temperature cycle took at least a week to change - Hawkins and Armstrong - Esther 1978

8 of 18

Circadian rhythms

Cognitive processing

Cognitive processing vary with the temperature rhythm.

Folkard et al 1977, found that long term memory recall in 12 to 13 year old children was best when they first learned the material at 3pm rather than 9pm.

In contrast to the cognitive high, alertness is best when body temperature is lowest, early morning. Alterness is associated with increase of cortisol which happens about an hour before walking.

9 of 18

Circadian Rhythms


These hormones all have circadian rhythms

  • Cortisol - peaks around 6 am
  • Melatonin  - peaks around midnight (produced when drak)
  • Growth hormone - peaks at midnight

Applications of this research

  • Since the circadian rhythm affects digestion, heart rate, hormone secretion and other functions, this should be taken into account when taking drugs.
  • This area is called chronotherapeutics.
  • Taking aspirin  to treat heart attacks which normally occur in the early morning is most effective around 11pm to allow the aspirin to peak in the blood stream which takes 2-4 hours.
10 of 18

Infradian Rhythms

Rhythms that last longer than one day.

Menstrual cycle - The menstrual cycle in human females is a set of changes in hormone functioning with a periodicity of about 28 days. It regulates fertility. Women can only conceive a child at a certain point in the menstrual cycle.

The human menstrual cycle is driven by fluctuating hormone levels. The function of this cycle is to regulate ovulation.

The fluctuating hormone levels are generated by the hypothalamus and affect:

  • Internal biology
  • Sexual behvaiour

Apparently influenced by zeitgebers

- Possible pheromonal mechanism

- Possible evoultionary advantage

11 of 18

Infradian Rhythms

Zeitgebers and the menstrual cycle

Sabbagh and Barnard 1984; The menstrual periods of women who spent a lot of time together often became synchronised, possibly due to a chemical secent - phermonone.

This could have had an evoultionary advantage - but the mechanisms of how it happens is as yet unclear.

Further Research

The menstrual cycle is governed by the release of hormones under the control of the pituitary gland. It can also be governed by other extranal factors. Women who live together. and do not take oral contraceptives, tend to menstruate at the same time each month. This shown by the research done by:

Russell et al 1967: Daily samples of sweat were collected from one group of women, and rubbed on the upper lips of other women. The women's cycles becaome synchronised with their individual odour donor, presumably because of pheromones in the sweat which affected the menstrual cycle.

12 of 18

Infradian Rhythms

Reinberg 1967 ; Found that the menstrual cycle of a woman who spent 3 months in a cave shortened. Reinberg speculated that this was due to low light levels.

However, not also research found support for the role of infradian rhythms in relation to the menstrual cycle constrasting research by.....

Miclintock - found no correlation between womens menstrual cycle patterns.

Male cycles

Males also have monthly rhythms.There is evdience of a periodic variation in both body temperature and subjective ratings of morning alertness, with a cycle length of approximately 20 days - Empson 1977.

13 of 18

Infradian Rhythms

Seasonal Affective Disorder- Application

  • SAD is a circannual rhythm - a type of infradian rhythm.
  • SAD is a depressive condition that pccurs yearly during the winter months.
  • The hormone melatonin is secreted by the pineal gland when it is dark. The more darkness, the more melatonin. Melatonin may be related to chronic depression.
  • Undersatnding SAD has led to the development of phototherapy. The sufferer is exposed to very strong lights to decrease the level of melatonin.
  • It is possible that SAD is the result of distrubance of circadian rhythms brought about by going to bed earlier, but waking up at the same time- resulting in the same symptoms of jet lag.
14 of 18

Ultradian Rhythms

Ultradian Rythms

Rhythms that are shorter than a day, the most well researched are thise occuring in sleep.

Sleep stages

In humans, daily cycles of wakefulness and sleep follow a circadian rhythm. BUT within the sleep portion of this cycle, there is an ultradian rhythm - the cycle of of NREM and REM sleep over the sleep period. Within each cycle - the person experiences NREM sleep followed by a shorter period of REM sleep.

EEGs show that the brain is active during sleep and that types of activity seemed to rlate to types of sleep.

15 of 18

Ultradian Rhythms

The sleep stages

  • Stage 1 = Drowsiness. Transition from being awkae to falling asleep. Brain waves and muscle activity begin to alow down.
  • Stage 2 = Period of light sleep during which eye movements stop . Brain waves become slower. Heart rate slows, and body temperature decreases.
  • Stage 3 = Extremely slow brain waves. Blood pressure falls and breathing slows.
  • Stage 4 = Stage of deepest sleep with the slowest brain waves. There is no eye movement but the muscles can still function, It is relatively difficult to be woken.
  • Stages 3 and 4 = are vital for restoring energy. This is also when thebody relases growth hormones.
  • Stage 5 = Active period of sleep marked by instense brain activity and dreams. Breathing is rapid and irregular. Heart rates and blood pressure fluctuate. Eyes move rapidly in various directions. The muscles are paralysed in REM sleep. Derment and Kleitman 1957 found that people woken during REM sleep usually reported dreaming. It is thought that the regions of the braon that are used in learning, thinking and organising information are stimulated during REM sleep.
16 of 18


The Multi Store Model of Memory

Atkinson and Shiffrin's 1968 - multi store model of memory is an explanation of how memory processes work. It is illustarted in the following diagram:

Sensory memory ---- Short term memory ---- Long Term memory

1. The stores  The frst thing multi store model says is that memory is made up of three different stores. A key thing about the multi store model of memory is that it states that these three atores are completely separate from one another.

Sensory Memory -The sensory registers everything our senses take in, that is everything we can hear, see, smell, and touch. It is likely that we have a sensory memory for each sense but so far only the iconic store.

Short term memory - The main features of STM have already been discussed. To briefyly recap, the stm uses acoustic encoding, has a capcity of 5-9 and duration of around 18 seconds without rehearsal.

17 of 18


Long Term Memory

Long Term Memory

The main features of LTM have also already been

18 of 18


No comments have yet been made

Similar Psychology resources:

See all Psychology resources »See all Biological Rhythms resources »