Circadian Rhythms (AQA A-Level Psychology): Revision Notes
Circadian Rhythms
Introduction to biological rhythms
Biological rhythms are distinct patterns of changes in body activity that follow cyclical time periods. These rhythms are controlled by two main factors: internal body clocks called endogenous pacemakers and external environmental changes known as exogenous zeitgebers.
All living organisms experience biological rhythms, which can be classified into different types based on their duration:
- Ultradian rhythms occur multiple times during the day
- Infradian rhythms take longer than a day to complete
- Circannual rhythms occur over much longer periods
- Circadian rhythms last approximately 24 hours
These different types of biological rhythms work together to regulate various physiological processes throughout our lives. Understanding these classifications helps researchers study specific patterns of biological activity and their underlying mechanisms.
What are circadian rhythms?
Circadian rhythms are a specific type of biological rhythm that follows roughly a 24-hour cycle. The term comes from the Latin words 'circa' (about) and 'diem' (day). These rhythms regulate numerous body processes, with the two most well-studied examples being the sleep/wake cycle and changes in core body temperature.
The natural tendency to feel drowsy at night and alert during the day demonstrates how daylight acts as an important exogenous zeitgeber, influencing our sleep/wake patterns.
Circadian rhythms don't just affect when we sleep - they coordinate essential functions including hormone production, digestion, blood pressure, and immune system activity throughout the 24-hour cycle.
The sleep/wake cycle
Our sleep/wake cycle provides clear evidence of how circadian rhythms operate. Most people naturally follow a pattern of sleeping when it's dark and waking when it's light, showing the influence of daylight as an external cue.
However, researchers have investigated what happens when biological clocks operate independently, without external stimuli like light - a condition referred to as free-running. If we had no awareness of whether it was day or night, would we still maintain regular sleep and wake times? Several studies have explored this question.
The concept of free-running rhythms is crucial for understanding the difference between our internal biological clock and the influence of environmental cues. This research helps separate nature from nurture in biological timing.
Key research studies
Siffre's cave study
Research Example: Siffre's Cave Study
Researcher: Michel Siffre (self-study) Participants: Siffre himself Aim: To investigate the effects of removing external time cues on biological rhythms Procedure: Siffre spent extended periods underground in caves, deprived of natural light and sound. He conducted two main studies - two months in the Southern Alps caves in September 1962 (believing it to be mid-August when he emerged), and later six months in a Texan cave. Findings: In both cases, his free-running biological rhythm settled to approximately 25 hours, though he continued to fall asleep and wake up on a regular schedule.
Aschoff and Wever (1976)
Research Example: Aschoff and Wever (1976)
Researchers: Jürgen Aschoff and Rütger Wever Participants: A group of participants Aim: To investigate circadian rhythms in controlled conditions Procedure: Participants spent four weeks in a WWII bunker deprived of natural light Findings: All but one participant displayed circadian rhythms between 24 and 25 hours
Folkard et al. (1985)
Research Example: Folkard et al. (1985)
Researchers: Simon Folkard and colleagues
Participants: 12 people
Aim: To test the strength of the free-running circadian rhythm
Procedure: Participants lived in a dark cave for three weeks, going to bed at 11:45pm and rising at 7:45am according to a clock. Researchers gradually speeded up the clock so that an apparent 24-hour day actually lasted only 22 hours.
Findings: Only one participant was able to comfortably adjust to the new regime, suggesting the existence of a strong free-running circadian rhythm that cannot easily be overridden by environmental changes.
Research conclusions
These studies suggest that the natural sleep/wake cycle may be slightly longer than 24 hours, but becomes entrained by exogenous zeitgebers associated with our 24-hour day (such as daylight hours, typical meal times, and social schedules). However, the influence of environmental cues on our internal biological clock should not be underestimated.
The consistent finding across studies that free-running rhythms tend to be around 25 hours, rather than exactly 24 hours, provides strong evidence for the existence of an internal biological clock that operates independently of environmental cues.
Evaluation
Practical applications
Shift work applications
Understanding circadian rhythms has improved knowledge of the adverse consequences that occur from their disruption, known as desynchronisation. Night workers engaged in shift work experience reduced concentration around 6am (a circadian trough), making mistakes and accidents more likely (Boivin et al., 1996). Research also indicates that shift workers are three times more likely to develop heart disease (Knutsson, 2003), partly due to the stress of adjusting to different sleep/wake patterns and poor quality sleep during the day.
This research has economic implications for managing worker productivity effectively.
Industries that rely heavily on shift work, such as healthcare, transportation, and manufacturing, have used this research to develop better rotation schedules and workplace lighting to minimise the negative effects of disrupted circadian rhythms.
Drug treatment applications
Circadian rhythms coordinate many basic body processes including heart rate, digestion, and hormone levels. This affects pharmacokinetics - how drugs are absorbed and distributed in the body. Research has revealed peak times when drugs are most effective, leading to guidelines for optimal timing of medications including anticancer, cardiovascular, respiratory, anti-ulcer and anti-epileptic drugs (Baraldo, 2008).
This field of study, called chronopharmacology, shows that the timing of drug administration can be just as important as the dosage. Some medications can be up to 40 times more effective when given at optimal times in the circadian cycle.
Methodological limitations
Use of case studies and small samples
Sleep/wake cycle studies typically involve small participant groups (as in Aschoff and Wever's study) or single individuals (like Siffre). These participants may not represent the wider population, limiting meaningful generalisations. Even Siffre's later cave experience in 1999 showed that at age 60, his internal clock operated much more slowly than when he was younger, illustrating how individual factors can vary and prevent general conclusions.
The reliance on small samples is a significant limitation because circadian rhythms can vary considerably between individuals due to genetic factors, age, and lifestyle differences. This makes it difficult to establish universal principles from limited research.
Poor experimental control
Although participants were deprived of natural light, they still had access to artificial light. Siffre, for example, used a lamp when awake that remained on until he went to bed. Early researchers assumed artificial light would have no effect on free-running biological rhythms. However, Charles Czeisler et al. (1999) demonstrated that participants' circadian rhythms could be adjusted from 22 to 28 hours using dim lighting. This suggests artificial light may act as a confounding variable, similar to participants taking a drug that resets their biological clock.
Modern research has shown that even very dim artificial light (as little as the light from a digital alarm clock) can influence circadian rhythms, highlighting how early studies may have underestimated environmental influences.
Individual differences
One complicating factor for generalising findings is that individual sleep/wake cycles can vary considerably, ranging from 13 to 65 hours (Czeisler et al., 1999). Research by Jeanne Duffy et al. (2001) revealed natural preferences, with some people preferring early bedtimes and rising ('larks') while others prefer the opposite ('owls'). Age differences in sleep/wake patterns also exist, making it difficult to establish universal principles from limited research samples.
These individual differences suggest that circadian rhythm research must account for genetic variations and personal chronotypes when developing applications for shift work management or medical treatments.
Remember!
Key Points to Remember:
- Circadian rhythms operate on approximately 24-hour cycles and regulate key processes like sleep/wake cycles and body temperature
- Endogenous pacemakers are internal body clocks, while exogenous zeitgebers are external environmental cues like daylight
- Research shows free-running rhythms tend to be slightly longer than 24 hours (around 25 hours) but become entrained to our 24-hour day
- Understanding circadian rhythms has practical applications for managing shift work and optimising drug treatments
- Studies face limitations including small samples, individual differences, and potential confounding variables like artificial light