Controlling Biological Rhythms

• What do we call parts of the brain that control the timing of the circadian rhythm?

  Pacemakers
• The pineal gland controls our sleep/wake cycle by releasing melatonin into the bloodstream. The more melatonin in the bloodstream, the more sleepy we feel. It increases the amount of melatonin released at night to make us sleep and reduces the amount of melatonin released in the day so that we feel awake and alert.
• What are the key factors of melatonin?


  • It is produced and released by the pineal gland
  • Its release decreases during the day to make us feel alert
  • Its release increases during the night to make us feel sleepy
• How does melatonin help to produce the sleep-awake cycle?


  • Melatonin release inhibits the wakefulness centres and activates the sleep centres.
  • Melatonin release is reduced during the day causing wakefulness centres to become active.
  • At night, melatonin is released from the pineal gland.
• What does SCN stand for?


  Suprachiasmatic nucleus
• The role of the suprachiasmatic nucleus?


  • The SCN is a group of neurons in the hypothalamus that controls the pineal gland through nerve impulses.
  • The SCN tells the pineal gland when to release releasing melatonin.
• Support for Endogenous Pacemakers: Ralph and the Hamsters


  • Ralph et al conducted a study investigating the sleep/wake cycle in hamsters.
  • Ralph et al had a control group with a normal sleep/wake cycle and an experimental group with an abnormal 21 hr sleep/wake cycle.
  • He first removed the SCN in both hamsters and then transported their SCN's in the opposite hamster.
• What is one limitation of Raph's (hamster) research?

  It may not generalise to humans.
• What are endogenous pacemakers?


  Structures within the body that control biological rhythms are called endogenous pacemakers.
• What are exogenous zeitgebers?


  Stimuli outside the body that influence biological rhythms are called exogenous zeitgebers.
• What are the three exogenous zeitgebers for the sleep/wake cycle?


  Noise, light and social customs
• Support for Exogenous Zeitgebers: Siffre and the Cave

  Siffre conducted a case study. He lived in a cave for 6 months with no access to natural light. He found that his sleep/wake cycle increased to 25-30 hour cycle without access to light.
• Support for Exogenous Zeitgebers: Aschoff & Weber


  Participants in Aschoff and Weber’s study were given lamps, but had no idea what time it was. Therefore they had to judge the time purely from their endogenous pacemakers. Aschoff and Weber found participants settled into longer sleep/awake cycle roughly 25-27 hours long.
• What is a limitation of Aschoff and Weber's study?


  The participants still had access to some artificial light, which may have helped reset their circadian rhythm. The results may have underestimated the importance of light, and overestimated the importance of endogenous pacemakers.
• Why does jetlag occur?

  When we travel to a new timezone, our endogenous pacemakers continue to operate in the old timezone, while our exogenous zeitgebers operate in the new timezone. (desychronised)
• How can bright light therapy help with jetlag?


  • A few days before people travel on a long distance flight, they’re exposed to artificial bright like to recreate the light levels in the place they’re flying to.
  • Exposure to artificial light helps people reset their circadian rhythms to the new timezone before they travel.
• What are the two limitations of Reinberg's study?

  • As it was a case study, we can't be sure how well the findings generalise to other women.
  • The artificial light of the small lamp may have helped reset the young woman's menstrual cycle
• Support for Exogenous Zeitgebers in the Menstrual Cycle


  Reinberg conducted a case study, in which he made a young woman live in a cave with no access to light to investigate her menstrual cycle. He found that living underground with no light caused the woman's menstrual cycle to shorten from 28 days to 25.7 days.