Biological Rhythms - The Circadian Rhythm Flashcards
Circadian Rhythms
Circadian rhythms are a 24-hour biological rhythm, often known as the ‘body clock’, which is reset by levels of light. They follow a natural, internal process that regulates, e.g., the sleep-wake cycle, and other physiological activities.
Examples of Circadian Rhythms
- The Sleep-Wake Cycle
- Body Temperature
- The Sleep-Wake Cycle
The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans and animals should be asleep and awake.
The exogenous zeitgeber light provides the primary input to this system, meaning that it is the external cue for sleeping or waking up. Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN), which is the endogenous pacemaker of the sleep-wake cycle. The SCN then uses this information to coordinate the activity of the entire circadian system.
However, sleeping and wakefulness are not determined by the circadian rhythm alone, but also by homeostasis. When an individual has been awake for a long time, homeostasis signals the body that there is a need for sleep due to energy consumption. This homeostatic drive for sleep increases throughout the day and reaches its maximum in the late evening, when most people fall asleep.
- Body Temperature
Body temperature is another circadian rhythm as it fluctuates over a 24-hour period. Human body temperature is at its lowest in the early hours of the morning (36 degrees Celsius at 4:30 am) and is at its highest in the early hours of the evening (38 degrees Celsius at 6 pm).
Sleep typically occurs when the core temperature starts to drop. The body temperature starts to rise toward the end of a sleep cycle promoting feelings of alertness first thing in the morning.
Strength
Point: There is existing research evidence supporting the role of external cues, such as light, in regulating circadian rhythms.
Evidence: Siffre conducted a self-experiment in which he lived in a cave without natural light or clocks for an extended period of time. When he emerged, he believed the date to be a month earlier than it actually was, highlighting that his perception of time had shifted. Essentially, his 24-hour sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it actually was, therefore leading him to believe that fewer days had passed.
Justification: This finding indicates that, in the absence of external cues, the body’s internal clock extends beyond the typical 24-hour cycle. This reflects the idea that circadian rhythms are not solely biologically determined but are influenced by exogenous zeitgebers, in this case light, which help reset the sleep-wake cycle.
Implication: This increases the practical utility of understanding circadian rhythms as the findings have real-world applications, such as in managing shift work and jet lag. Understanding the role of external cues in regulating biological rhythms can help develop strategies, like exposure to bright light at strategic times, to minimise disruptions to the sleep-wake cycle and overall well being.
Counterargument: However, Siffre’s case study has been criticised for its lack of generalisability, as it involved only one participant - himself, therefore raising concerns about whether the findings are applicable to the wider population.
Evidence: Individual differences, such as Siffre’s age, lifestyle, or genetic makeup, could have influenced his circadian rhythm, making it difficult to apply the findings to the general population. For example, later in life, Siffre repeated the study and found that his internal clock slowed down, suggesting that circadian rhythms may vary depending on factors like age.
Justification: This means that his original findings may not have been universally applicable as it makes the incorrect assumption that absence of exogenous zeitgebers, such as light, increases the duration of the circadian rhythm for everybody when it has proved to vary depending on differing factors, e.g., age.
Implication: This lowers the population validity of this study in relation to circadian rhythms as it fails to account for individual differences, therefore the findings cannot be confidently generalised to everyone.
Counterargument: Despite concerns about generalisability, research by Aschoff and Weber provide support for Siffre’s conclusions, and strengthens the argument that circadian rhythms extend beyond 24 hours without external cues.
Evidence: In their study, participants lived in a bunker without natural light but could control artificial light. The results showed that most participants developed a sleep-wake cycle of around 25-27 hours, similar to Siffre’s findings.
Justification: This indicates that Siffre’s results were not purely due to individual differences but reflect an apparent biological tendency for the circadian rhythm to extend in the absence of light.
Implication: This increases the reliability of research into circadian rhythm as it exemplifies that these patterns are not unique to one individual but apply more broadly and generally to humans. Such findings reinforce the importance of light as a key exogenous zeitgeber and have practical applications, such as improving artificial lighting environments to better support sleep-wake cycles in workplaces, hospitals and travel spaces.
Weakness
Point: A limitation of research into the circadian rhythm is that it tends to overlook individual differences, as not everyone follows the same 24-hour sleep-wake cycle.
Evidence: Duffy et al. found that individuals have natural preferences for different sleep-wake patterns. Their research identified ‘morning people’, who prefer to wake up and go to bed early (around 6am - 10pm), and ‘evening people’, who prefer to wake up and go to bed later (around 10am - 1am).
Justification: This demonstrates that circadian rhythms are not identical for everyone and may be influenced by genetic or environmental factors. The existence of such variation challenges the idea that a single, universal circadian rhythm applies to all individuals, meaning research findings may not always extensively apply to all humans.
Implication: This decreases the validity of most research in assessing how the circadian rhythm works because they neglect important individual differences. By acknowledging these variations, future research can develop more tailored approaches to sleep and work schedules, benefiting areas such as shift work, productivity and mental health.
Weakness
Point: A limitation of research into circadian rhythms is that it has traditionally focused on light as the primary factor influencing biological clocks, potentially overlooking the role of other important variables, such as temperature.
Evidence: Buhr et al. found that fluctuations in body temperature help regular circadian rhythms by setting the timings of cells in the body, influencing when tissues and organs become active or inactive. They argued that light information is converted into neural signals that then regulate body temperature, meaning temperature itself may play a crucial role in controlling biological rhythms.
Justification: This demonstrates that circadian rhythms are influenced by multiple interaction factors, rather than being solely dictated by light exposure. Since even small changes in body temperature can have significant effects on our internal clock, temperature may be just as - if not more - important than light in regulating biological rhythms.
Implication: This highlights the need for a more holistic approach to circadian rhythm research, considering multiple biological and environmental influences. Future studies should investigate how light, temperature and other factors interact, which could lead to better strategies for managing sleep disorders and improving overall well-being.
Strength
Point: A major strength of circadian rhythm research is that it relies on objective and scientific methods.
Evidence: Studies often use EEGs, melatonin level measurements, and core body temperature recordings, which provide quantifiable and replicable data. Unlike self-reports, which can be subjective, these biological measures give reliable insights into how circadian rhythms operate.
Justification: The use of scientific technology means findings are empirical and reproducible, immensely contributing to psychology’s status as a scientific discipline.
Implication: This enriches the scientific rigour of research into circadian rhythms as they rely on tangible, quantitative evidence instead of subjective speculations. As a result, research findings can be confidently applied in real-world settings, such as sleep therapy.
