Circadian rhythms

Circadian rhythms AO1

• A cycle that occurs once every 24 hours

• Eg sleep-wake cycle

• Influenced by exogenous zeitgebers (external influences eg sound, temp & noise) & endogenous pacemakers (internal influences eg hormones) which act as a blended system

Michael Siffre aim AO1

Top investigate the natural duration of the sleep-wake cycle when exogenous zeitgebers are removed

Michael Siffre method AO1

• He lived alone in a cave for six months 

• He had no natural light, no clocks, no radio, and no contact with the outside world

• His body temperature, sleep–wake times, and psychological state were recorded & he decided when to sleep and wake based purely on how he felt (no schedule imposed).

Michael Siffre findings AO1

• Initially his bodily rhythms were very erratic, but they soon settled into fairly regular sleep-wake cycles of 25 hours 

• When he emerged from isolation, he had ‘lost’ 28 days (February)

• In later life, Siffre noted his body clock slowed further, suggesting age-related changes in circadian rhythms

Michael Siffre conclusion AO1

• Endogenous pacemakers play a role in controlling circadian rhythms as he maintained a consistent daily cycle, even in the absence of exogenous zeitgebers 

• Exogenous zeitgebers also play a role as his 24 hour sleep-wake cycle was increased by the lack of external cues (eg light & sound)

Circadian rhythms strengths AO3

• P - practical applications

• E - research has found that shift workers experience a period of reduced concentration at around 6 a.m., when body temperature and alertness are lowest. This makes them more likely to make mistakes or have accidents, especially in sectors like healthcare & industry. These outcomes not only endanger lives but also result in significant financial costs to employers and the wider economy through lost productivity, compensation claims, and healthcare expenses. Also, long-term has been found to increase the risk of heart disease, possibly due to chronic sleep deprivation and stress caused by circadian disruption

  T - economic implications as employers can optimise shift patterns to fit better with human biological rhythms — for example, by introducing forward-rotating shifts, strategic napping, or controlled light exposure to help workers stay alert during night shifts. These interventions can reduce fatigue-related errors and improve efficiency, leading to higher productivity and lower economic losses. Furthermore, healthier, better-rested employees are less likely to develop long-term conditions linked to disrupted circadian rhythms (e.g. cardiovascular disease or depression), which reduces healthcare costs and sickness-related absences. Therefore, research has practical value by improving workforce performance, public safety, and national productivity.

Circadian rhythms limitations AO3

• P - poor control in Michael Siffre’s study

• E - although he was isolated from daylight, he used artificial electric light to help him see and maintain a normal routine. He turned the light on when he woke up and off when he went to bed, assuming it would have no effect on his biological clock. However, later research (Czeisler et al) found that even very dim artificial light (as low as 100 lux) can act as a powerful exogenous zeitgeber, capable of resetting the circadian rhythm. This means that the artificial light in Siffre’s cave acted as a confounding variable, because it may have synchronised his internal clock, rather than leaving it to run purely on its endogenous pacemaker

• T - the findings that Siffre’s sleep–wake cycle extended to around 25 hours may not truly reflect the natural, free-running rhythm of the human body & could instead represent a rhythm that was partly influenced by light exposure, which undermines the study’s internal validity

• P - research into circadian rhythms use small samples

• E - Michael Siffre studied himself & he may not be representative of the wider population

• T - limits the extent to which findings can be generalised. Also, there may be significant individual differences in circadian rhythms as research shows that biological rhythms vary depending on age, gender, lifestyle, and genetic factors. Even Siffre later reported that his biological rhythm slowed with age, meaning his results from earlier experiments were not even consistent within the same person. Therefore, relying on small or unique samples makes it hard to know whether the results represent typical human behaviour or are simply idiosyncratic (unique to the individual). Also, many few people will be willing to live in a cave for 6 months, meaning it is unlikely the study can be replicated to check for consistencies, therefore meaning reliability is low. 

• P - individual differences

• E - Research shows that humans vary in their chronotype and internal cycle length. For example, Duffy et al. (2001) found that younger adults tend to be evening types (“owls”) with later peaks in alertness, whereas older adults are typically morning types (“larks”) with earlier sleep–wake

• T - indicates that circadian research cannot assume a uniform 24–25-hour cycle for all humans. Even if studies used larger samples, results would still vary because people naturally differ in their sleep–wake patterns, peak alertness, and responses to environmental cues. Individual differences highlight that circadian rhythms are flexible and influenced by personal factors, limiting the general applicability of research findings