Biological rhythms Flashcards
AO1: What is a biological rhythm?
Biological rhythms have an important influence on the way in which body systems behave.
All biological rhythms are controlled by endogenous pacemakers (internal body clock) and influenced by exogenous zeitgebers (external changes in the environment)
AO1: Difference between the rhythms
Ultradian rhythms – This is a cycle that is less than 24 hours e.g. the stages of sleep which last approximately 90 minutes.
WHEREAS
Circadian Rhythms – This is a cycle that lasts for 24 hours e.g. body temperature, and the sleep wake cycle
WHEREAS
Infradian rhythms – This is a cycle that lasts longer than 24 hours e.g. the female menstrual cycle which operates approximately on a 28 day cycle
AO1: The Effect of Endogenous Pacemakers and Exogenous Zeitgebers on the Sleep Wake Cycle
Circadian rhythms is a cycle that lasts for 24 hours e.g., the sleep wake cycle.
Circadian rhythms are controlled by endogenous pacemakers, internal factors like the internal body clock, to maintain biological rhythms. The main endogenous pacemaker is called the Suprachiasmatic Nucleus.
Circadian rhythms are also influenced by exogenous zeitgebers which are external factors which are in our environment which can influence our sleep/wake cycle through entrainment, such as light, which resets the SCN suggesting our sleep/wake cycle is not only influenced by endogenous pacemakers (internal factors).
The sleep wake cycle is controlled by the master endogenous pacemaker, the suprachiasmatic nucleus (SCN). Our eyes notice a change in light as it gets dark and less light is received by the retina. This sends information to the SCN which stimulates the pineal gland to release melatonin and promote sleep. When our eyes detect light again (exogenous zeitgeber) the SCN is reset which stimulates the SCN and pineal gland, which inhibits the release of melatonin to promote wakefulness. This suggests the sleep wake cycle is controlled by endogenous factors.
What are the evaluations for endogenous pacemakers and exogenous zeitegebers on the sleep wake cycle? (4)
1) RTS SCN connections in chipmunks
2) Discussion: Animal bias
3) RTS Michel Siffre - cave
4) Lacks ecological validity
AO3: RTS SCN connections in chipmunks
Research to support the effect of endogenous pacemakers on the sleep wake cycle was conducted by DeCoursey et al. They destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days. The sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators. This may be because they were awake and vulnerable to attack when they should have been asleep. This supports the influence and importance of endogenous pacemakers, such as the SCN on the maintenance of the sleep wake cycle, as exogenous zeitgebers alone were not able to maintain the sleep wake cycle of the chipmunks. Therefore, increasing the validity of research into the role of endogenous pacemakers and exogenous zeitgebers in maintaining the sleep wake cycle/circadian rhythms
AO3: Discussion: Animal bias
However, research investigating the impact of endogenous pacemakers on the sleep wake cycle can be criticised for animal bias. This is potentially a problem because humans may have more a complex interaction between endogenous pacemakers and exogenous zeitgebers as there are a greater number of zeitgebers that can influence rhythms in humans such as social interaction and clocks, compared to an animal’s instinctive responses to limited external factors. Therefore, there are issues with extrapolating the findings of research to humans, which limits the extent to which DeCoursey’s findings support the importance of endogenous pacemakers on the sleep wake cycle/circadian rhythm.
AO3: RTS Michel Siffre - cave + discussion
Research to support the role of endogenous pacemakers on the sleep wake cycle was conducted by Michel Siffre. He lived in a cave isolated from all exogenous zeitgebers e.g., clocks and the light dark cycle for six months. It was found that within a few days he had developed a consistent 25-hour cycle and continued to fall asleep and wake up on a regular schedule. This suggests that the sleep wake cycle is controlled by an endogenous pacemaker due to Siffre developing a consistent rhythm, when separated from external cues, showing it is primarily governed by endogenous pacemakers rather than exogenous zeitgebers. Therefore, increasing the validity of research into the role of endogenous pacemakers and exogenous zeitgebers in maintaining the sleep wake cycle/circadian rhythms.
DISCUSSION: However, some may argue that because Siffre’s sleep wake cycle did adjust to longer than 24 hours, that exogenous factors are important in maintaining the sleep wake cycle.
AO3: Lacks ecological validity
However, research, such as Siffre’s, lacks ecological validity as Siffre’s living conditions were highly controlled and so may not reflect how endogenous pacemakers and exogenous zeitgebers interact in real life. Only in exceptional circumstances are endogenous pacemakers free-running and unaffected by the influence of exogenous zeitgebers. This demonstrates that pacemakers and zeitgebers work together to maintain the sleep wake cycle and it makes little sense to separate the two to argue which has the greater impact. Therefore, this reduces the external validity of the findings and cannot be easily generalised to explain how endogenous pacemakers impact the sleep wake cycle/circadian rhythms in real life.
