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Biological Rhythms (Sleep)

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Biological Rhythms Circadian Rhythms A circadian rhythm is a biological rhythm that runs on a roughly 24 hour cycle. One example is the sleep/wake cycle. Humans sleep about 8 out of 24 hours. Even under constant light conditions, animals keep a circadian rhythm (e.g. sleeping and eating) of around 24 hours. This suggests that circadian rhythms are endogenous (internal) as they continue to run even when exogenous (external) zeitgebers are missing. However, it seems that we rely on some exogenous zeitgebers (such as light) to entrain our circadian rhythms with a 24 hour day, otherwise they may become slightly out of sync. The main endogenous pacemaker is the suprachiasmatic nucleus (SCN), a tiny cluster of neurons located in the hypothalamus in the brain. It causes the pineal gland to secrete melatonin (a hormone that makes us sleepy) in response to low light conditions. However, even in constant light conditions the free-running SCN still controls melatonin secretion to a roughly 24 hour cycle. One strength of explanations of circadian rhythms is that they suggest a relationship between biology and the environment. For example, they consider how the (biological) SCN works with the exogenous zeitgeber light to control the sleep/wake cycle. As most psychologists agree that both of these factors usually play a part, a combination of the two is sensible. ...read more.


This is due to an imbalance of hormones. It is thought the menstrual cycle is controlled by an (internal) endogenous pacemaker, mainly the pituitary glad. However, research has shown that it can be entrained to the environment the women it in by certain external (exogenous) factors, such as pheromones. One strength of the idea that exogenous pacemakers such as pheromones can affect infradian rhythms like the menstrual cycle is that it is supported by the evolutionary approach. This means that it has been shown to have an evolutionary advantage. For example McClinktock found women working in all male environments had shorter menstrual cycles. He concluded this gave them more opportunities to reproduce and pass on their genes. This suggests an evolutionary advantage to an endogenous infradian rhythm (menstruation) being affected by an exogenous factor (male pheromones). However, one criticism of the idea that behaviours instigated by the infradian rhythm of PMS affect all women is that it is deterministic. This means that it suggests that all women will become 'automatons' of their hormones when they have PMS. For example, although some women display aggression when they experience PMS, not all women do. This is a problem because by suggesting that all women become automatons, the idea ignores the human characteristic of free will. Evidence to support the idea that infradian rhythms can be affected by exogenous pacemakers comes from Russell et al. ...read more.


This is positive as most psychologists a combination of external and internal factors is sensible. The explanations can therefore be praised for taking a more holistic approach. One issue with many explanations of ultradian rhythms is that they are nomothetic. This means they assume that everyone experiences sleep in the same way. For example, in Southern Europe and South America people take 'siestas' in the afternoon, so their sleep pattern may be different due to altered sleeping hours. Therefore a more idographic approach (focused on the individual) would be more suitable so by taking a nomothetic approach explanations may be too simplistic. Evidence supporting the idea that exogenous zeitgebers can affect the endogenously controlled ultradian rhythm of sleep stages comes from Dement and Wolpert. In a sleep laboratory, they sprayed REM sleepers in the face with water. They found that those sprayed reported dreaming of water more frequently than those who were not sprayed. This is positive because it seems water (an exogenous zeitgebers) can influence the ultradian rhythm of sleep stages. One problem with this research is that it was conducted in the artificial setting of a laboratory. This means that the setting did not reflect the participants' natural environment. For example, sleeping in a laboratory may have caused the participants to have more disturbed sleep. This is a problem as this may have caused the results to be misleading, meaning the results may have lacked internal validity. ...read more.

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