Biological rhythms

• Biological rhythms exert an important influence on the way in which body systems behave. They are cyclical in their pattern.

  • They are influenced by both endogenous pacemakers and exogenous zeitgebers.
• Endogenous Pacemakers: These are internal biological clocks, such as the suprachiasmatic nucleus (SCN) in the hypothalamus, which regulate rhythms like the sleep-wake cycle.
• Exogenous Zeitgebers: These are external environmental cues, such as light, temperature, or social cues, that influence and entrain our biological rhythms to align with the environment.
• Ralph et al. (1990) conducted a study on biological rhythms to investigate the role of the suprachiasmatic nucleus (SCN).

  • Procedure: Ralph and his team used mutant hamsters with a naturally shorter circadian rhythm of around 20 hours. They then transplanted the SCN from these hamsters into normal hamsters with a 24-hour circadian rhythm.
  • Findings: The recipient hamsters adopted the 20-hour rhythm, indicating that the circadian rhythm was controlled by the SCN.
• Campbell and Murphy (1998) investigated the role of light as an exogenous zeitgeber in regulating the sleep-wake cycle.

  • Procedure: Participants were exposed to light on the back of their knees during the night, bypassing their eyes.
  • Findings: This exposure shifted their circadian rhythms by up to 3 hours, demonstrating that light can influence biological rhythms through pathways other than the eyes.
• The three main types of biological rhythms:

  • Circadian Rhythms (≈ 24 hours)
  • Infradian Rhythms (> 24 hours)
  • Ultradian Rhythms (< 24 hours)
• Circadian rhythms are biological rhythms that follow a 24-hour cycle e.g. the sleep-wake cycle.
• The sleep-wake cycle is governed by both endogenous pacemakers and exogenous zeitgebers.

  • Endogenous Pacemakers: The suprachiasmatic nucleus (SCN) (a small part of the hypothalamus) acts as the body's internal clock. It regulates melatonin production in the pineal gland.
  • Exogenous Zeitgebers: Light is an external cue. Darkness triggers melatonin release, making us sleepy, while daylight suppresses melatonin, keeping us awake.
• Siffre's Cave study (1975): Procedure

  • lived in a cave in Texas for 6 months, deprived of natural light, clocks or other time cues
  • had food and drink and was allowed to perform daily activities like reading and exercising
  • a telephone connected him to researchers above ground, but he could only contact them when he woke up and went to sleep - without knowing the actual time
  • his body temperature, heart rate, and cognitive functions were also monitored
• Siffre's cave study (1975): Findings

  • His sleep-wake cycle initially followed a 24-hour pattern but gradually extended to approximately 25 hours.
  • This meant he would wake up, eat, and sleep at later times each day, believing that fewer days had passed than in reality
  • This suggested that the natural circadian rhythm is slightly longer than 24 hours (around 25 hours) when deprived of external cues like light
• Siffre's cave study (1975): Conclusions

  • The study supports the role of endogenous pacemakers (internal biological clocks like the suprachiasmatic nucleus, SCN) in regulating the sleep-wake cycle.
  • However, in normal conditions, exogenous zeitgebers (e.g., sunlight) help to reset and entrain our rhythm back to exactly 24 hours.
  • Without external cues, the sleep-wake cycle still persists but drifts slightly beyond 24 hours.
• Strengths of Siffre's Cave study (1975):

  • High internal validity: The study was in a controlled environment, ensuring no external cues influenced Siffre’s biological rhythm.
  • Real-world applications: Findings help in understanding jet lag, shift work, and space travel, where external time cues are disrupted.
  • Supports role of endogenous pacemakers: The fact that his rhythm persisted without external cues suggests that internal biological mechanisms.
• Weaknesses of Siffre's Cave study (1975):

  1. Small sample size:

  • Involved only one person so findings may lack generalisability to the wider population.
  • Individual differences exist - people may have shorter or longer circadian rhythms naturally.

  1. Artificial conditions:

  • He was not completely isolated from external factors - his contact with researchers and use of artificial lighting may have influenced his body clock.

  1. Conflicting evidence:

  • Some studies have found that circadian rhythms vary between individuals (Czeisler et al. (1999) found circadian rhythms could range from 13 to 65 hours).
• Aschoff & Wever (1976):

  • Placed participants in a WWII bunker without clocks or natural light.
  • Found that most participants developed a natural cycle of 24-25 hours, showing circadian rhythms persist without external cues but may drift slightly.
• Infradian rhythms are biological rhythms that last longer than a day (24 hours) e.g. menstrual cycle and seasonal affective disorder (SAD).
• The menstrual cycle is a infradian rhythm that is regulated by hormonal changes, lasting around 28 days.

  • Endogenous control: Hormones (oestrogen & progesterone) control ovulation.
  • Exogenous influences: McClintock’s (1998) research suggests that pheromones from other women can synchronize cycles.
• McClintock & Stern (1998) research into infradian rhythms:

  • Collected pheromones from 29 women and applied them to others.
  • 68% of women experienced cycle changes (found cycles sychronised)
  • Suggests pheromones act as exogenous zeitgebers.
• Seasonal affective disorder (SAD) is a type of depression linked to seasonal changes, typically in winter.

  • Thought to be due to a lack of sunlight affecting melatonin production.
  • Treatment: Light therapy, which regulates melatonin and serotonin levels.
• Ultradian rhythms are biological rhythms that last less than 24 hours, so occur more than once per day e.g. the sleep cycle.
• The Sleep Cycle

  • A complete cycle lasts 90 minutes and repeats multiple times a night
  • Consists of five stages
• The five stages of the sleep cycle:

  • stage 1 and 2: light sleep (sleep escalator) - alpha and theta waves, participant easily woken
  • stage 3 and 4: deep sleep (slow wave sleep) - delta waves, participant is difficult to wake, essential for physical recovery
  • stage 5: rapid eye movement (REM) - associated with dreaming and characterised by movement inhibition and a sensory blockade, brain activity is similar to waking state
• Dement & Kleitman (1957)

  • Procedure: Monitored sleep using EEGs (brain scans); controlled alcohol and caffeine intake of participants to remove effects of extraneous variables.
  • Findings:
  • Found discrete periods of rapid eye movement potentials were recorded.
  • REM sleep is linked to dreaming.
  • Participants awoken during REM were able to accurately recall their dreams.
  • Supports the existence of distinct sleep stages.
• One strength of research into circadian rhythms is the strong supporting evidence from Siffre’s cave study. Siffre (1975) spent six months in a cave without natural light or time cues, yet his sleep-wake cycle remained regular but extended to around 25 hours. This suggests that the circadian rhythm is controlled by endogenous pacemakers (e.g. SCN) but is usually synchronized to 24 hours by external cues (exogenous zeitgebers, like light). This supports the idea that biological rhythms are governed by both internal and external factors, making research into circadian rhythms highly valid.
• Research into circadian rhythms has important real-world applications in understanding shift work and developing medical treatments. Studies (e.g. Bolvin et al) show that night shift workers experience reduced concentration around 6 am, increasing the risk of accidents and health issues. This has led to practical applications, such as adjusting shift work patterns and improving chronotherapeutics (timing medication for when it’s most effective). This highlights the usefulness of circadian rhythm research in improving health and workplace safety.
• A limitation of circadian rhythm research is that many studies fail to control extraneous variables. Siffre was still exposed to artificial light (lamp) which could have reset his biological clock. Czeisler et al. (1999) found that artificial light can shift circadian rhythms, contradicting earlier assumptions that only natural light had an effect. This suggests that previous studies may not have measured the true effects of endogenous pacemakers, reducing the validity of the findings. Future research should control for artificial light exposure to provide more reliable conclusions.
• One limitation is that much of the research into circadian rhythms lacks ecological validity and generalizability. Studies such as Siffre’s cave study and Aschoff & Wever’s bunker study involved small sample sizes, meaning the findings may not be representative of the wider population. Individual differences exist in circadian rhythms—some people have natural variation (e.g., "larks" vs. "owls"), which means findings may not apply to everyone. Therefore, while these studies provide valuable insight, they may lack population validity, limiting how widely they can be applied.