6.4.1 Circadian, Infradian and Ultradian 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
  • Endogenous 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
• Melatonin plays a role in ‘triggering’ sleep by responding to a decrease in the levels of light
• 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

Circadian rhythms determine our sleep-wake cycle

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

Siffre (1973) during his cave study research

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

Link to Approaches:

• This topic sits within the biological approach which assumes that human behaviour can be explained in terms of internal physiological functions such as the activation of the pineal gland in the sleep-wake cycle linked to hormones such as melatonin 
• However, human behaviour does not always fit neatly into a biological explanation, for example some people will not necessarily respond to a pre-determined sleep-wake cycle which means that the biological approach cannot always account for individual differences

• 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
• SAD is 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, 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 

Seasonal affective disorder is an example of an infradian rhythm

The stages of sleep: an example of an ultradian rhythm

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 

A participant in a sleep study

• Mc Lintock et al.’s (1998) research was a field study which means that an array of extraneous variables
  • For example, diet, exercise, pollution, could have interfered with the findings
• Dement and Kleitman carried out their research in the 1950s which means that it may lack temporal validity
  • Temporal validity is to what extent research study findings can be generalised and applied to a current time or situation
  • For example, the use of digital technology such as smartphones, which did not exist at the time of the research, may be interfering people’s ultradian rhythms
  • This means new research should be undertaken on this topic

Link to Issues and Debates:

• 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