The Effect of Endogenous Pacemakers & Exogenous Zeitgebers (AQA A Level Psychology)

The effect of endogenous pacemakers & exogenous zeitgebers on the sleep/wake cycle

Endogenous pacemakers

• Biorhythms are regulated by two key factors

  • endogenous pacemakers

  • exogenous zeitgebers 

• Endogenous pacemakers are the internal mechanisms which determine the biological rhythms of the body e.g.

  • the circadian sleep-wake cycle 

• Endogenous pacemakers act as internal ‘body clocks’ but, they can be affected by the external environment e.g.

  • changes in light levels (e.g. when the day turns to dusk)

  • light is required to reset the sleep-wake cycle every 24 hours (Siffre's research supports this, although he found a natural 25-hour cycle)

• The suprachiasmatic nucleus (SCN) is the primary endogenous pacemaker, closely associated with the pineal gland

  • The SCN is the body’s ‘home-made’ method for inducing sleep

  • It is located in the anterior hypothalamus, with links to other areas of the brain responsible for sleep and arousal (e.g. the pineal gland, the optic nerve and the photosensitive ganglion cells of the retina)

  • The SCN also receives information about light levels to ensure that biorhythms synchronise with the outside world, e.g. day and night

  • The SNC sends signals to the pineal gland, which increases production of melatonin secretion at night, helping to induce sleep

Research into endogenous pacemakers

• Morgan (1995):

  • the researcher bred hamsters so that their circadian rhythms were 20 hours, rather than the usual 24-hour cycle

  • SCN neurons from the 20-hour hamsters were transplanted into the brains of normal hamsters,

  • the hamsters with transplanted SCN neurons went on to follow the same irregular 20-hour circadian rhythm as the original 20-hour cycle hamsters

  • thus, the transplanted SCN had imposed its irregular pattern onto the hamsters

    • the researcher concluded that the SCN is a significant endogenous pacemakers which is crucial to the proper functioning of circadian rhythms

    • these findings also demonstrate the importance of exogenous zeitgebers to the sleep-wake cycle (endogenous pacemakers and exogenous zeitgebers work hand-in-hand)

Evaluation of research into endogenous pacemakers

Strengths

• The use of carefully controlled conditions and removal of the SCN means that this study has good internal validity

  • The researcher was able to directly observe the effect of circadian rhythm disruption in a way which would have been impossible with human research

• Further support for the influence of endogenous pacemakers on circadian rhythms was provided by Skene & Arendt (2007)

  • They pointed out that most blind people who still have some light perception have normal circadian rhythms of 24 hours, whereas blind people with no light perception display irregular circadian rhythms

  • This demonstrates the importance of light in terms of the internal sleep-wake cycle

Limitations

• Morgan's research is flawed due to its use of hamsters as subjects

  • Humans are very different biologically, cognitively and socially from hamsters

  • This means that research using hamsters is not generalisable to humans

  • The role of endogenous pacemakers in humans therefore cannot be explained using animal research

• It is biologically reductionist to focus on a narrow range of explanations when considering something as complex and prone to variation as the patterns and habits involved in the sleep-wake cycle

  • There are likely to be an array of other factors which also contribute to circadian rhythms which may supersede the endogenous pacemaker explanation e.g.

    • high-stress levels may interfere with regular sleep

    • some people can (and may frequently) fall asleep in bright light, or during daylight hours which means that light may not be as important to the sleep-wake cycle as some research suggests

Exogenous zeitgebers

• Exogenous zeitgebers are any factors in the sleep-wake cycle which act as external cues to either wakefulness or sleepiness e.g.

  • 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

  • trying to sleep during the day - even if the person is very tired - may not happen due to light levels

  • someone may sleep through their alarm clock if it has been set to an unusually early time (their body is not used to waking at this time so their sleep may be very deep)

• Exogenous zeitgebers are responsible for resetting the biological clock of an organism

• Light is the main and most important exogenous zeitgeber, in the same way as it is crucial to the functioning of endogenous pacemakers

• Social cues also act as exogenous zeitgebers e.g.

  • the timing of meals (e.g. 'We eat dinner at 7pm every evening')

  • work schedules (e.g. 'I get to work at 8.30am and leave at 4.30pm)

  • exercise sessions (e.g. 'Five-a-side football happens every Tuesday at 6pm')

• People can compensate when there is a lack of natural light using these social cues instead 

• Synchrony occurs between the internal clock (endogenous pacemakers) and external cues (exogenous zeitgebers) e.g.

  • rising levels of light in the morning which triggers wakefulness

  • the alignment of behaviours such as waking, eating meals, working etc. to a regular sleep-wake cycle is known as entrainment

• Thus, endogenous pacemakers and exogenous zeitgebers interact and work alongside each other to set circadian rhythms

Research into exogenous zeitgebers

• Campbell & Murphy (1998):

  • 15 sleeping participants were woken at various intervals throughout the night

  • A light pad was shone onto the back of their knees

  • This resulted in irregular sleep-wake cycles for the participants (as high as three hours deviation for some of them)

  • The researchers concluded that light is a key exogenous zeitgeber - even when it shines on the back of the knee rather than being detected by the eye

Evaluation of research into exogenous zeitgebers

Strengths

• Campbell & Murphy's study presents compelling evidence of the role of light as an exogenous zeitgeber

  • This means that the argument that exogenous zeitgebers have a direct effect on the sleep-wake cycle has some validity

Limitations

• A sample size of 15 is very small

  • This means that the researchers are limited as to how generalisable and robust their findings are

    • Generalisability is limited due to the unrepresentative nature of a small sample

    • The robustness of data is limited as small numbers lack statistical power

• Campbell & Murphy's findings have not been replicated in any subsequent studies

  • If a study cannot be replicated according to a standardised procedure and conditions then it cannot be checked for consistency

  • Not being able to check for consistency of findings means a lack of reliability