Biological rhythms are cycles with the body which are controlled by endogenous pacemakers and exogenous zeitgerbers e.g. light and social cues. Circadian rhythms are biological cycles that last 24 hours e.g. the sleep/wake cycle. The main biological clock in mammals is the suprachiasmatic neucleus (SCN) which aids in keeping biological rhythms synchronised with the outside world through regular events such as meal times.
The sleep wake cycle is governed by external cues e.g. light and social cues but there is also an endogenous clock which is free running, working without external cue. It sets a rhythm at about 24-25 hours. The external cues help the endogenous clock to remain synchronised with the outside world. Studies have shown circadian rhythms persist despite isolation from natural light, but are not accurate highlighting the importance of exogenous zeitgerbers.
Core body temperature is another circadian rhythms. It is it lowest around 4:30am at 36 degrees c and highest at 6pm at 38 degrees c with a slight dip occurring at midday, even without eating.
Hormone production also follows a circadian rhythm with cortisol levels being lowest around midnight and highest around 6am. Cortisol plays a role in making us alert therefore explains why awoken in the middle of the night we struggle to function. Melatonin and growth hormone also follow a circadian rhythm both peaking at around midnight.
Aschoff and Wever found participants to have circadian rhythms between 24 and 25 hours, when placed in an underground, with some as long as 29 hours. Rhythms persisted in the absence of external cues supporting the assumption of circadian rhythms being internal. Limitations of this study include low external validity as it was conducted in a laboratory setting therefore findings cannot be generalised to real life. However, it could be argued in a real life setting absence of external cues would never be an issue. But, such studies can be used to help gain further understanding of how our circadian rhythms work. However, participants may have shown demand characteristics as they were aware they were being monitored affecting the reliability and validity of results.
Michel Siffre spend 6 months in a cave without external cues and found his circadian rhythms varied from a 25-30 hours. This highlights the presence of an internal circadian clock but also highlights the importance of exogenous zeitgebers to regulate rhythms as his circadian rhythms were not maintained at 24 hours. However, results from a single case study may not provide accurate generalisations on how others would respond to a lack of external cues. Confounding variables such as temperature, air pressure and being connected to monitoring equipment may have affected sleep quality and length, therefore reliability and validity of results may be questioned. Also results suffer gender bias and it cannot be assumed women's circadian rhythms would have similar responses due to hormonal differences.
Research findings have positive real world application particularly in the area of chronotherapeutics, which recent research in Germany has found, to influence the effectiveness of drugs therapies. For example aspirin is effective in reducing the chance of a heart attack if taken in the evening as most attacks occur in the early hours of the morning. Therefore research into circadian rhythms may help to increase effectiveness of treatments for disorders and illnesses.
Both Aschoff and Wevers's and Siffre's studies suffer the issue of the use of abnormal environments. This may have affected length and quality of sleep, acting as a confound variable therefore affecting the validity of findings. Additionally, participants in both studies were able to us bright artificial lights which other research has found to cause rhythms to shift, further affecting the studies internal validity. Such research can be used to highlight the existence of free running circadian clocks. However, they are not accurate therefore suggesting the importance of interaction with external zeitgebers to maintain circadian rhythms.
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