Circadian rhythms are biological cycles lasting 24hours like the sleep/wake cycle, which is facilitated by time-checks and regular events such as mealtimes (external cues). The main internal biological clock (circadian rhythm) in mammals appears to be located in the hypothalamus, which is responsible for “motivation” and is named the suprachiasmatic nucleus (SCN). The SCN has an inbuilt circadian firing pattern as when this has been damaged in rats, the circadian rhythm involving sleeping and feeding patterns has also been disrupted (Zucker et al). The SCN regulates the secretion of melatonin in the pineal gland (another endogenous pacemaker which produces melatonin which affects sleep) and is also connected to the retina of the eye through a separate pathway. This highlights the indirect link between exogenous zeitgebers such as light and how melatonin production from the pineal gland (endogenous pacemakers) works together with the SCN to maintain the circadian rhythm. Light can also reach the brain via other means as Campbell et al demonstrated how it was possible to reset the circadian rhythm through shining light on participants knees. This shows other secondary oscillators exist throughout the body maintaining circadian rhythms through the use of exogenous zeitgebers.
Core body temperature is another circadian rhythm which sees its lowest point at 4:30am (36 degrees) and highest at around 6pm (38 degrees). A slight trough occurs after lunch and this dip occurs even when people do not eat.
Hormone production also follows a circadian rhythm with cortisol at its lowest around midnight and peaking at 6am. Cortisol plays a role in making us alert and explains why if awaken at 4am we struggle to think clearly. Melatonin and growth hormone also have a circadian rhythm with both peaking at midnight.
Circadian Rhythm Strengths/Weaknesses and Evaluation
Aschoff and Weaver placed participants in a bunker without any external cues and found participants to have circadian rhythms between 24-25 hours though some were as high as 29 hours. This demonstrated the existence of circadian rhythms and their endogenous pacemakers (internal clocks), which persisted even without exogenous zeitgebers to influence them. This also highlighted the importance of external cues as these internal clocks were not accurate without them. Due to the lab setting this may have low ecological as it is not indicative of real world settings for sleep behaviour. Also low external validity as this may have then affected the quality or quantity of sleep participants had due to the artificial setup. The sample was also small meaning generalization is more difficult to the wider population where differences may be more evident on a bigger scale. The participants were also volunteers who were aware of being monitored on sleep patterns, which may have caused demand characteristics and affected sleep patterns resulting in results which lack validity and not measuring what the study was meant to measure effectively.
Michel Siffre spent 6 months in a cave without external cues and found his circadian rhythm varied from 25-30 hours again highlighting the existence of an internal circadian clock. This also highlighted the importance of exogenous zeitgebers in regulating internal biological clocks. However this was a single case study involving one individual and such generalisations may not apply to others due to individual differences. Age may also have been a factor as results may also be limited to a similar age group as other studies have shown sleep patterns vary among different age groups. Other factors such as temperature, air pressure or even the use of monitoring equipment could affect the quality of results meaning this study lacks internal validity. For example artificial light was used and may have been a confounding variable and Campbell et al and Cziesler et al (1999) showed even this can manipulate circadian rhythms through artificial light. Therefore such a study may have low external validity to real world settings but also low internal validity and not actually measure what it was intending to measure (the absence of light or external cues) due to this. Such experimental studies are important however as they allow us to demonstrate causal relationships.
Duffy et al also found a case for individual differences in circadian rhythms. Morning people preferred to rise early and go to bed early (6am and 10pm) while evening people prefer going to sleep and wake up late (10am and 1am) showing peoples circadian rhythms could vary from one another.
Zucker’s study where he damaged the SCN in rats to disrupt circadian rhythms was an animal study and may not apply to humans due to differences in anatomy. Therefore it may lack external validity and generalisation in humans. There are also ethical concerns when it comes to intentionally harming such animals although others may argue the benefits gained in understanding animal biology may lead to further understanding of humans. Such studies are typical of the biological approach to understanding human behaviour. They propose behaviour can be explained due to biological structures in the brain or hormonal activity. In truth our behaviour is much more complex and not so deterministic as such biological explanations propose. “Nurture” is evidently a strong factor too with environmental influences and exogenous zeitgebers clearly having a strong role in overriding internal biological clocks to some degree. On the other hand Miles et al demonstrated how a blind man who had a circadian rhythm of 24.9 hours struggled to reduce his internal pace no matter what exogenous zeitgebers were used highlighting some biological clocks may be more ingrained and not influenced.
The SCN is evidently not the only biological clock as other studies have shown that there are other oscillators in the body that appear to regulate biological rhythms through other means (temperature, light penetrating other parts of the body) and explaining circadian rhythms as simply dictated by the SCN and pineal gland connection is oversimplifying the workings of human biology which is far more complex.
Understanding circadian rhythms has real world applications particularly in the field of Chronotherapeutics. This is the study of how timing affects drug treatments and as the circadian rhythm affects digestion, heart rate and hormones among other functions, this can be taken into account when consuming drugs. For instance medicine that affect certain hormones may have no effect if taken when the target hormone level is low but more effective if taken when they are high. Aspirin for example is most effective in treating heart attacks and most effective if taken in the late evening as most attacks occur in the early hours of the morning.
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How to reference/cite/link to this article:
<a href=”https://www.loopa.co.uk/circadian-rhythm-biological-rhythms-aqa-psychology/”>Circadian Rhythms – Biological rhythms</a>