Biological Rhythms


Biological Rhythms

Biological rhythms are behaviours which are repeated periodically. These behaviours are controlled by either endogenous pacemakers (internal biological clocks) or by exogenous zeitgabers (external/environmental cues).

Key terms:

  • Circadian rhythms - bodily cycles occuring every 24 hours.
  • Infradian rhythms - bodily cycles occuring less than once a day.
  • Ultradian rhythms - bodily cycles occuring more than once a day.
  • Endogenous pacemakers - internal body clocks regulating biological rhythms,
  • Exogenous zeitgabers - external stimuli involved in the control of biological rhythms.
  • Biological rhythms - regular patterns of physiological, cognitive and behavioural activity occuring as circadian, unltradian and infradian rhythms.
  • Shift work - periods of work performed outside regular employment hours.
  • Jet lag - temporary disruption of biological rhythms caused by high-speed travel across time zones.

Circadian Rhythms

  • Circadian rhythms are biological cycles lasting around 24 hours, like the sleep/wake cycle, which is usually facilitated by time-checks and regular events such as meal times.
  • There is a free running cycle controlled by an endogenous pacemaker working as an internal body clock.

Research for circadian rhythms:

  • Aschoff and Weber (1962) - they placed participants in a bunker with no natural light and they settled into a sleep/wake cycle of between 25 and 27 hours, suggesting that endogenous pacemakers control the sleep/wake cycle in the absence of light cues and that light seems necessary to coordinate the biological clock.
  • Siffre (1975) - spent 6 months in a cave with no time cues. Artificial lights came on when he was awake, he settled into a sleep/wake cycle of 25 to 30 hours. After 179 days he thought that 151 days had passed, supporting Aschoff and Weber's findings that endogenous pacemakers exert and influence on circadian rhythms, thought the use of artificial light may have been a confounding variable.
  • Folkard et al (1985) - isolated 12 participants from natural light for 3 weeks, manipulating the clock so that only 22 hours passed a day. 12 out of the 11 participants kept place with the clock, showing the strenght of the circadian rhythm as a free-running cycle.

Evaluation for circadian rhythms:

  • Individual difference exist in the sleep/wake cycle; cycle length of individual's vary and the timing of these cycles also varies. Duffy et al (2000) found that early risers perfer 6am to 1pm and later risers prefer 10am to 1am. Aschoff and Weber (1976) found in isolation studies that some participants maintain normal cycles, where as some strongly differ.
  • Isolation studies normally have few participants, which makes generalisation problematic.
  • Research suggests that endogenous pacemakers do exist and are regulated by exogenous zeitgabers, for example, light control.
  • Isolating participants from the environment without controlling their behaviour may be insufficient to reveal the activity of the endogenous pacemaker.
  • Czeisler et al (1999) suggest that participants in earlier sleep/wake cycles were inadvertnetly affected by exposure to high levels of artificial light, skewing the results.
  • Czeisler et al also claim that the cycle is actually 24 hours. In their 1999 study, 24 men and women lived for a month in


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