Biological Rhythms

• Circadian rhythms (aka the ‘body clock’) operate on a 24-hour cycle which is reset by levels of light.
• The sleep-wake cycle is an example of a circadian rhythm

  Humans and animals will sleep or wake according to this cycle, dependent on the amount of light (an exogenous zeitgeber) they are exposed to

  Exogenous zeitgebers are environmental events, like the changing of light, which reset the body clock.
• The suprachiasmatic nuclei (SCN) in the hypothalamus detects the level of light present and then uses this information to coordinate the activity of the entire circadian system.
• Shift work has been found to lead to desynchronisation of circadias rhythms and can lead to adverse cognitive and physiological effects

  For example, memory lapses or issues with fertility

  Jet lag is another example that can lead to desynchronisation of circadian rhythms.
• desynchronization is often used to refer to the disconnection or disruption of systems or processes that were previously synchronized.
• Supporting research:

  Siffre (1973) – a case study in which Siffre spent two months in a cave deprived of light and sound, determining that his circadian rhythm remained between 24-25 hours

  Folkard et al. (1980) implemented a 22-hour sleep-wake cycle and found that participants experienced problems with recall in their short-term memory
• Critique:

  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 are the menstrual cycle and seasonal affective disorder (SAD)
• A typical menstrual cycle takes around 28 days to complete and is the result of a change in hormone levels including oestrogen.
• Ultradian rhythms occur more than once within a 24-hour cycle, for example, the cycles of sleep.
• There are five recognised stages of sleep

  Stages 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.
• Supporting research:

  McLintock et al. (1998): Infradian rhythms – a 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

  Dement and Kleitman (1957): Ultradian rhythms – participants in a sleep lab were attached to an EEG monitor which showed that REM sleep was correlated with dreaming.
• 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 (e.g. to her fertility, to her emotional state) which the researchers would not be able to predict.
• The rhythms of our body are regulated by two key factors - endogenous pacemakers and exogenous zeitgebers
• Endogenous pacemakers are the internal mechanisms which determine the biological rhythms of the body

  For example, the circadian sleep cycle

  They are known as the internal ‘body clocks’ but, they can be affected by the external environment

  For example, changes in light
• The suprachiasmatic nucleus is a major factor in the sleep/wake cycle - it is the most important endogenous pacemaker

  It is the body’s ‘home-made’ method for inducing sleep

  Specific biological triggers are involved which result in someone falling asleep
• The stages of sleep: an example of an ultradian rhythm

  
• Exogenous zeitgebers are any factors in the sleep-wake cycle which act as external cues to either wakefulness or sleepiness
• The body can be entrained so that its internal mechanisms are altered by these artificial external cues - exogenous zeitgebers

  Going to bed with the light on or a digital device placed next to the bed may keep levels of light unnaturally high and thus impact negatively on sleep

  Light is the main and most important exogenous zeitgeber .
• Social cues also act as exogenous zeitgebers, for example:

  The timing of meals

  Work schedules

  Exercise sessions

  People can compensate when there is a lack of natural light using these social cues instead
• These exogenous zeitgebers (the lamp, the digital clock, the phone) may interrupt the body’s natural sleep-wake cycle

  
• Supporting research:

  Morgan (1995): 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

  Campbell and Murphy (1998): 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
• Critique:

  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

  The sample size used in Campbell and Murphy’s (1998) study was 15 which is too small to be able to generalise from, meaning that the results lack external validity
• The argument for endogenous pacemakers and exogenous zeitgebers as key players in the sleep-wake cycle is the emphasis on the role of light in the processes involved

  However, 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
• Which two of the following are implicated in the functioning of endogenous pacemakers:

  AO1 = 2 marks

  1. Light

  b) Melatonin

  c) Social Cues

  d) SAD

  e) The pineal gland

  the correct response is b) and e) as these are both ‘made in the body’ which is what ‘endogenous’ means. Options a) and c) are exogenous zeitgebers; d) is an example of an infradian rhythm.