biological rhythms

• Circadian rhythms


  The 'body clock' that operates on a 24-hour cycle and is reset by levels of light
• Sleep-wake cycle


  An example of a circadian rhythm
• Exogenous zeitgebers


  Environmental events, like the changing of light, which reset the body clock
• Suprachiasmatic nuclei (SCN)


  Detects the level of light present and then uses this information to coordinate the activity of the entire circadian system
• Melatonin


  Plays a role in 'triggering' sleep by responding to a decrease in the levels of light
• Shift work


  Leads to desynchronisation of circadian rhythms and can lead to adverse cognitive and physiological effects
• Adverse effects of desynchronised circadian rhythms

  • Memory lapses
  • Issues with fertility
• Jet lag is another example that can lead to desynchronisation of circadian rhythms
• Siffre spent two months in a cave deprived of light and sound, determining that his circadian rhythm remained between 24-25 hours


  1973
• Folkard et al. implemented a 22-hour sleep-wake cycle and found that participants experienced problems with recall in their short-term memory


  1980
• Case studies such as Siffre (1973) cannot be generalised beyond the single participant
• Folkard et al.'s (1980) research lacks ecological validity due to its lab setting
• Infradian rhythms


  Have less than one cycle in 24 hours
• Examples of infradian rhythms

  • Menstrual cycle
  • Seasonal affective disorder (SAD)
• Menstrual cycle


  A typical cycle takes around 28 days to complete and is the result of a change in hormone levels including oestrogen
• Seasonal affective disorder (SAD)


  Usually experienced during the darker months of winter when more melatonin is secreted which may result in higher levels of depression
• Ultradian rhythms


  Occur more than once within a 24-hour cycle
• Examples of ultradian rhythms


  • Cycles of sleep
• Stages of sleep


  • Stage 1 and 2 are the sleep escalator
  • Stages 3 and 4 consist of the delta waves of deep sleep
  • Stage 5 is REM sleep, the stage in which dreaming occurs according to research
• McLintock et al.'s longitudinal field study in which the menstrual cycles of women became synchronised after being exposed to a 'donor' pheromone which acted as an exogenous zeitgeber


  1998
• Dement and Kleitman's research in which participants in a sleep lab were attached to an EEG monitor which showed that REM sleep was correlated with dreaming

  1957
• McLintock et al.'s (1998) research was a field study which means that an array of extraneous variables could have interfered with the findings
• Dement and Kleitman carried out their research in the 1950s which means that it may lack temporal validity
• New research should be undertaken on the topic of ultradian rhythms due to the potential impact of digital technology on sleep patterns
• Comparison of biorhythms


  • Circadian: Every 24 hours, Example: Sleep-wake cycle, Research: Siffre (1973)
  • Infradian: Less than one cycle in 24 hours, Example: Menstrual cycle, Research: Mc Lintock et al. (1998)
  • Ultradian: More than one cycle in 24 hours, Example: Sleep stages, Research: Dement and Kleitman (1957)
• There may be ethical considerations involved in conducting research on infradian rhythms as seen in McLintock et al.'s study (1998): deliberately disturbing a woman's natural menstrual cycle could have far-reaching effects
• Endogenous pacemakers


  The internal mechanisms which determine the biological rhythms of the body
• Exogenous zeitgebers


  Any factors in the sleep-wake cycle which act as external cues to either wakefulness or sleepiness
• Suprachiasmatic nucleus (SCN)


  The most important endogenous pacemaker, it is the body's 'home-made' method for inducing sleep
• Light is the main and most important exogenous zeitgeber
• Social cues that act as exogenous zeitgebers

  • Timing of meals
  • Work schedules
  • Exercise sessions
• Morgan removed the SCN of hamsters and found that their sleep-wake cycle disappeared but then re-appeared once fetal hamster SCN cells had been transplanted into their brains


  1995
• Campbell and Murphy demonstrated that light (in the form of a torch) is a key exogenous zeitgeber, even when shone on the back of participants' knees, as it disrupted their sleep cycles by up to three hours


  1998
• The use of animals in research (see Morgan, 1995 above) could be argued to be unethical if the procedures involved cause physical and/or mental harm to the animal
• Campbell and Murphy's (1998) study had a small sample size of 15 which is too small to be able to generalise from, meaning that the results lack external validity
• Some people are able to (and may frequently) fall asleep in bright light, during daylight hours, etc. which means that light may not be as important to the sleep-wake cycle as some researchers suggest
• It may be overly reductionist to focus on a narrow range of explanations when considering something as complex and variable as the patterns and habits involved in the sleep-wake cycle