6.4.2 The Effect of Endogenous Pacemakers & Exogenous Zeitgebers

• The rhythms of our body are regulated by two key factors - endogenous pacemakers and exogenous zeitgebers 
• Endogenous pacemakers are the internal mechanisms which determine the biological rhythms of the body
• For example, the circadian sleep cycle 
• They are known as the internal ‘body clocks’ but, they can be affected by the external environment 
• For example, changes in light 
• The suprachiasmatic nucleus is a major factor in the sleep/wake cycle - it is the most important endogenous pacemaker 
• It is the body’s ‘home-made’ method for inducing sleep
• Specific biological triggers are involved which result in someone falling asleep
  
  

The stages of sleep: an example of an ultradian rhythm

• Light issues the first signal to begin the sleep cycle
• Siffre’s (1973) ‘caveman’ study demonstrates that humans tend to fall into, roughly, a 24-hour sleep-wake cycle (his own cycle was measured as 25 hours)

• His findings are evidence of an internal biological ‘clock’ determining the sleep-wake cycle

• Exogenous zeitgebers are any factors in the sleep-wake cycle which act as external cues to either wakefulness or sleepiness 
• The body can be entrained so that its internal mechanisms are altered by these artificial external cues - exogenous zeitgebers 
•  Going to bed with the light on or a digital device placed next to the bed may keep levels of light unnaturally high and thus impact negatively on sleep
• Light is the main and most important exogenous zeitgeber 
• Social cues also act as exogenous zeitgebers, for example:
• The timing of meals
• Work schedules
• Exercise sessions
• People can compensate when there is a lack of natural light using these social cues instead 

These exogenous zeitgebers (the lamp, the digital clock, the phone) may interrupt the body’s natural sleep-wake cycle

Supporting research:

• Morgan (1995): removed the SCN of hamsters and found that their sleep-wake cycle disappeared but then re-appeared once fetal hamster SCN cells had been transplanted into their brains
• Campbell and Murphy (1998): demonstrated that light (in the form of a torch) is a key exogenous zeitgeber, even when shone on the back of participants’ knees, as it disrupted their sleep cycles by up to three hours

Critique:

• The use of animals in research (see Morgan, 1995 above) could be argued to be unethical if the procedures involved cause physical and/or mental harm to the animal
•  The sample size used in Campbell and Murphy’s (1998) study was 15 which is too small to be able to generalise from, meaning that the results lack external validity 

Link to Approaches

As well as being part of the Biological Approach exogenous zeitgebers also have relevance to the Behaviourist Approach as the inclusion of social cues as triggers in the sleep-wake cycle draws from schedules of reinforcement which is a feature of operant conditioning.

Social cues such as meal times can act as exogenous zeitgebers

Link to Issues and Debates:

• The argument for endogenous pacemakers and exogenous zeitgebers as key players in the sleep-wake cycle is the emphasis on the role of light in the processes involved 
• However, some people are able to (and may frequently) fall asleep in bright light, during daylight hours, etc. which means that light may not be as important to the sleep-wake cycle as some researchers suggest
• It may be overly reductionist to focus on a narrow range of explanations when considering something as complex and variable as the patterns and habits involved in the sleep-wake cycle