One biological rhythm is the 24-hour circadian rhythm (often known as the ‘body clock’), which is reset by levels of light.
The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans and animals should be asleep and awake. Light provides the primary input to this system, acting as the external cue for sleeping or waking. Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN). The SCN then uses this information to coordinate the activity of the entire circadian system. Sleeping and wakefulness are not determined by the circadian rhythm alone, but also by homoeostasis. When an individual has been awake for a long time, homeostasis tells the body that there is a need for sleep because of energy consumption. This homeostatic drive for sleep increases throughout the day, reaching its maximum in the late evening, when most people fall asleep.
Body temperature is another circadian rhythm. Human body temperature is at its lowest in the early hours of the morning (36oC at 4:30 am) and at its highest in the early evening (38oC at 6 pm). Sleep typically occurs when the core temperature starts to drop, and the body temperature starts to rise towards the end of a sleep cycle promoting feelings of alertness first thing in the morning.
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Research Support: Research has been conducted to investigate circadian rhythms and the effect of external cues like light on this system. Siffre (1975) found that the absence of external cues significantly altered his circadian rhythm: When he returned from an underground stay with no clocks or light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was, and leading to his thinking that fewer days had passed.
Individual Differences: However, it is important to note the differences between individuals when it comes to circadian cycles. Duffy et al. (2001) found that ‘morning people’ prefer to rise and go to bed early (about 6 am and 10 pm) whereas ‘evening people’ prefer to wake and go to bed later (about 10 am and 1 am). This demonstrates that there may be innate individual differences in circadian rhythms, which suggests that researchers should focus on these differences during investigations.
Additionally, it has been suggested that temperature may be more important than light in determining circadian rhythms. Buhr et al. (2010) found that fluctuations in temperature set the timing of cells in the body and caused tissues and organs to become active or inactive. Buhr claimed that information about light levels is transformed into neural messages that set the body’s temperature. Body temperature fluctuates on a 24-hour circadian rhythm and even small changes in it can send a powerful signal to our body clocks. This shows that circadian rhythms are controlled and affected by several different factors and suggests that a more holistic approach to research might be preferable.
Discuss research into circadian rhythms. Refer to evidence in your answer (16 marks)
Circadian rhythms are a type of biological rhythm subject to a 24-hour cycle, which regulates several body processes such as the sleep/wake cycle and changes in core body temperature. The sleep/wake cycle is controlled by exogenous zeitgebers which are external cues that may affect or entrain our biological rhythms such as the influence of light- which affects melatonin levels. Several studies have researched into the circadian rhythm. This includes studies done by Siffre, Aschoff+ Wegener and Folkard et al.
In Siffre’s study he stayed in a cave for an extended amount of time (6 months) without access to a clock, natural light (only having access to a lamp) or sound to see if his Circadian rhythm was affected. In Siffre’s case his ‘free-running’ biological rhythm settled down to one that was beyond the usual 24 hours (around 25 hours) though he did continue to fall asleep and wake on a regular basis. Additionally, Siffre resurfaced in mid-September (1962) believing it to be mid-August. Another study completed was by Aschoff and Wever (1976) who convinced a group of participants to spend 4 weeks in a WW11 bunker deprived of natural light. All but one of the participants (whose cycle extended to 29 hours) displayed a circadian rhythm between 24-25 hours. Both Siffre’s experience and the bunker study suggest that it is entrained by exogenous zeitgebers associated with our 24-hour day (e.g. number of daylight hours, typical mealtimes etc…). Furthermore Folkard et al. studied a group of 12 people who agreed to live in a dark cave for 3 weeks, retiring to bed when the clock said 11.45 pm and rising when it said 7.45. Over the course of the study, the researchers gradually speeded up the clock (unbeknown to the participants) so an apparent 24 hours eventually lasted 22 hours. It was revealed that not one of the participants was able to comfortably adjust to new rhythm. This therefore suggests that the existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in the external environment.
One limitation of research into circadian rhythms is that they don’t account for individual differences. A large amount of information into circadian rhythms are based on case studies. By nature, case studies are very specific and zoomed into to a certain individual or set of people. Therefore, it is hard to generalise findings from one person to a wider population. Thus, results from Siffre’s study have low ecological validity as Siffre, like every person, is unique and will have some aspects about him that others do not. For example, Siffres age, gender and health may have had an impact on the results of his study, and so these individual differences cannot be applied to larger population. Moreover, Duffy et al found that ‘morning people’ prefer to wake up and go to sleep early whereas evening people like to wake and sleep later. This illustrates that there may be innate individual differences in the circadian rhythm, which suggests that researches should be aware of these differences during an investigation. This is a limitation as case studies are one off atypical events that may not provide population validity or be able to be generalised to everyone.
A strength of research into circadian rhythms is that it has practical application to drug treatments. Circadian rhythms coordinate the body’s basic processes (e.g. heart rate, hormone levels). This, in turn, influences pharmacokinetics which is the action of drugs on the body and how well they are absorbed and distributed. Research into circadian rhythms has revealed that there are certain peak times during the day or night when drugs are likely to be at their most effective. This has led to the development of guidelines to do with the timing of drug dosing for a whole range of medications including anti-ulcer and anti-epileptic drugs. This is strength of circadian rhythms as it has a good real-life application and can have a positive effective on people through, this is as they allow for there to be peak times for things such as drug use.
Additionally, it has been suggested that temperature may be more important than light in determining circadian rhythms. Buhr et al found that fluctuations in temperature set the timing of cells in the body and caused tissues and organs to become active or inactive. Buhr claimed that information about light levels is transformed into a neural message that set the body’s temperature. Body temperature changes thought the 24-hour circadian rhythm and even the smallest of changes can act as an influential signal to our body clock. Therefore, this illustrates that circadian rhythms are controlled and affected by several difference aspects and suggests that a more holistic approach would be more desirable. This is a limitation as it conveys how not all research into circadian rhythms prove unanimous results and some researches may emphasise things more than others.