Biological rhythms

• Biological rhythms are regular cyclical changes in our biological systems.
• Rhythms repeat over different lengths of time
• All biological rhythms are governed by 2 things:
  • 1) the body's internal biological 'clocks' = endogenous pacemakers 2) external changes in the environment = exogenous zeitgebers
• Biological rhythms vary in length:
  • Circadian rhythms (CR) have cycles that generally occur once every 24 hours - for instance the sleep/wake cycle
  • Infradian rhythms (IR) have cycles that occur less than once a day - for instance the mentrual cycle
  • Ultradian rhythms (UR) have cycles that occur more than once every 24 hours - for example the sleep cycle has several repeating stages of light and deep sleep

• Circadian rhythms are those rhythms that last for around 24 hours
• This includes the the sleep/wake cycle
• When we feel drowsy at night and alert during the day shows the effect of daylight = daylight is an important exogenous zeitgeber on our sleep/wake cycle
• What if there was no influence of an external stimuli such as light on our biological clock?

• Siffre spent several extended periods underground to study the effects on his own biological rhythms
• He was deprived of exposure to natural light and sound
• After 2 months in the caves, he believed it to be mid-August 
• His study shows how his free running biological rhythm (BR) settled down to one that was just beyond the usual 24 hours (around 25 hours) 
• He also did continue to fall asleep and wake up on a regular schedule

• Aschoff and Wever found similar results to Siffre
• They convinced a group of participants to spend 4 weeks in a world war 2 bunker
• Deprived of natural light
• Results: all but one of the participants displayed a circadian rhythm between 24 - 25 hours. 
• Siffre and the bunker study suggest that the natural sleep/wake cycle may be slightly longer than 24 hours but that it is entrained by exogenous zeitgebers associated with our 24 hour day

• Folkard et al studied a group of 12 people who agreed to live in a dark cave for 3 weeks
  • going to bed when it was 11:45pm and waking up when it was 7:45am
  • The researchers gradually speeded up the clock so an apparent 24 hour day eventually lasted only 22 hours 
  • Results: only one of the participants was able to adjust to the new regime
  • This shows the existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in the external environment

• There was some practical application to shift work. Circadian rhythms gives researchers better understanding of the consequences that can occur as a result of their disruption.
• Night workers engaged in shift work experience a period of reduced concerntration around 6am meaning mistakes and accidents are more likely to occur
• Research also suggests the relationship between shift work and poor health - shift workers are 3x more likely to develop heart disease which may be due to the stress of adjusting to different sleep/wake patterns and the lack of poor quality sleep during the day
• Research into the sleep/wake cycle may have economic implications in terms of how best to manage worker productivity

Practical application to drug treatments

• Circadian rhythms co-ordinate a number of the body's basic processes such as heart rate, digestion and hormone levels
• This can affect the action of drugs in the body and how well they are absorbed and distributed
• Research  into circadian rhythms shows that there are certain peak times during the day or night when drugs are more likely to be most effective
• This then led to the development of guidelines of when to take a drug during a day

Practical application to drug treatments

• Circadian rhythms co-ordinate a number of the body's basic processes such as heart rate, digestion and hormone levels
• This can affect the action of drugs in the body and how well they are absorbed and distributed
• Research  into circadian rhythms shows that there are certain peak times during the day or night when drugs are more likely to be most effective
• This then led to the development of guidelines of when to take a drug during a day

Use of case studies and small samples

• The people involved in Aschoff and Wever / Siffre's study may not be representative of the wider population and this limits the extent to which meaningful generalisations can be made
• Siffre conducted another study, at the age of 60, that his internal clock ticked much more slowly than when he was a young man. 
• This shows that even with the same man involved, there are factors that vary which may prevent general conclusions being drawn

Poor control in studies

• Even though the p's were deprived of natural light, they still had artifical light
• In Siffre's study, he turned on a lamp everytime he woke up which remained on until he went to sleep. 
• He assumed that arficial light has no effect on the free-running biological rhythm
• However, Czeisler et al were able to adjust p's circadian rhythm from 22 to 28 hours using dim lighing

Individual differences

• Generalisations being made, when in fact peoples cycles can vary
• Duffy et al revealed that some people display a natural preference for going to bed early and rising early
• Some people choose to do the opposite
• There are also age differences in sleep/wake patterns

• Studies that have deprived humans of natural light have still allowed artifical light, which may give many of the benefits of natural light - this reduces the validity of these studies as in Siffre's study