Circadian

• A cycle that occurs once every 24 hours.


  Circadian Rhythm
• 1. The sleep wake cycle
  2. Core body temperature - highest around 4pm and lowest around 4am
  3. Hormone production:

  • Melatonin and growth hormone - both peak at midnight
  • Cortisol - at its lowest at midnight and peaks around 9am.

  Examples of Circadian Rhythms
• Evidence to support for the idea that the SCN is the key endogenous pacemaker in the sleep-wake cycle comes from Morgan (1995).

  • If the SCN is removed from the hamsters their CR is removed entirely.
  • When it is transplanted from foetal hamsters it returns completely.
  • Shows clear cause and effect relationships between presence of SCN and the maintenance of CR.


  AO3 Of Circadian Rhythms
• If the SCN transplanted was mutated with a shorter cycle of 20 hours, then the hamster will adopt those activity patterns. Shows circadian rhythms are built into the SCN.


  Morgan (1995)
• The main Endogenous pacemaker involved in circadian rhythms are said to be the Suprachiasmatic Nucleus (SCN).
• The SCN can respond to EZs like light. However even without light, the SCN still controls a rise and fall in melatonin production suggesting both EPs and EZs play a part in controlling these rhythms.


  Exogenous Zeitgebers in Circadian Rhythms
• • Spent periods underground in texas for 6 months.
  • His biological rhythm settled into a ‘free-running’ rhythm of around 25 hours.

  Siffre study
• • Participants spent 4 weeks in a WWII bunker, deprived of natural light.
  • All but one participants displayed a cirdadian rhythm between 24 and 25 hours - suggests that siffres study can be generalised to other people and that EPs and EZs are required to maintain a 24 hourly rhythm.

  Aschoff and Wever - supporting Siffre
• • 12 people lived in a cave.
  • Researchers speeded up the clock so a 24-hour day lasted 22 hours.
  • No participants were able to adjust to the new regime. - suggests that our circadian rhythms may only be controlled to a certain extent and that endogenous mechanisms (e.g. SCN) are the driving force behind circadian rhythms.

  Folkard et al
• • If you shine light on the back of ppts knees, it shifts their biological rhythm.
  • Implies that natural light may play a similar role in entraining our biological clocks to keep the sleep-wake cycle in synchrony with the outside world.


  Campbell and Murphy (1998)
• • Employers can use this research to organise shift working patterns to increase worker productivity.
  • Suggests ongoing research is important to better understand how to help shift workers adapt to changing daily patterns, which can benefit workers lives as well as the economy.


  Many practical applications - Circadian rhythms