endogenous pacemakers + exogenous zeitgebers

• what is the sleep/wake cycle?

  a daily cycle of biological activity based on a 24 hour period (circadian rhythm) that is influenced by regular changes in the environment, such as the alternation of night and day
• what is an endogenous pacemaker?

  internal body clocks that regulate biological rhythms
• what is the endogenous pacemaker for the sleep/wake cycle?

  superchiasmatic nucleus (SCN)
• what is the evidence for the SCN being the endogenous pacemaker in the sleep/wake cycle?

  the SCN (bundle of cells in hypothalamus) lies above the optic chiasm where nerve fibres from nasal side of each visual field cross to go to visual cortex in opposite brain hemispheres - the SCN receives input about light/dark from cells in optic chiasm, so SCN can adjust circadian rhythms like sleep/wake cycle to any changes in pattern of daylight
• what is an exogenous zeitgeber?

  external stimuli that affect or entrain our biological rhythms
• what is the exogenous zeitgeber for the sleep/wake cycle?

  light
• what is the evidence for light being the exogenous zeitgeber in the sleep/wake cycle?

  without external cues, the free-running biological clock continues to produce sleep/wake cycles. zeitgebers adjust the cycle to be in line with the external environment e.g. day and night, so control of the cycle is due to an interaction between internal + external factors
• what is entrainment?


  using an exogenous zeitgeber to adjust (or control) a circadian rhythm
• what is the SCN?

  suprachiasmatic nucleus
• what was the aim of the study into the SCN as an endogenous pacemaker by decoursey et al (2000)?


  to investigate the role of the SCN in the sleep/wake cycle in chipmunks
• what is the procedure of the study into the SCN as an endogenous pacemaker by decoursey et al (2000)?

  the SCN connections to the optic chiasm and pineal gland were cut in the brains of 30 chipmunks. they were then returned to their natural habitat and observed for 80 days
• what were the results of the study into the SCN as an endogenous pacemaker by decoursey et al (2000)?

  the sleep/wake cycle of the chipmunks completely disappeared and many were killed by predators because they'd been awake when they should've been asleep
• what was the conclusion of the study into the SCN as an endogenous pacemaker by decoursey et al (2000)?

  the SCN is important in establishing and maintaining the sleep/wake cycle
• what was the aim of the study into the SCN as an endogenous pacemaker by ralph et al (1990)?

  to investigate the role of the SCN in the sleep/wake cycle in hamsters
• what is the procedure of the study into the SCN as an endogenous pacemaker by ralph et al (1990)?

  mutant hamsters who had a 20 hour sleep/wake cycle were bred and the SCN tissue from the brains of foetus mutant hamsters was transplanted in to the brains of normal hamsters
• what were the results of the study into the SCN as an endogenous pacemaker by ralph et al (1990)?

  the sleep/wake cycle of the second group of previously 'normal' hamsters changed to 20 hours
• what was the conclusion of the study into the SCN as an endogenous pacemaker by ralph et al (1990)?

  the SCN is important in establishing and maintaining the sleep/wake cycle
• explain the mechanism by which the SCN maintains the sleep/wake cycle

  the SCN passes info on a day length to the pineal gland which during the night produces the hormone melatonin that induces sleep. melatonin production is inhibited by daylight, so the individual wakes up
• what is a negative evaluation point for research into the role of the SCN as an endogenous pacemaker in the sleep/wake cycle?

  ethics - decoursey et al destroyed SCN connections in chipmunks + then returned them to their habitat where changes in sleep/wake cycle led to them being killed

  reductionist - every cell in the body has its own pacemaker, influenced by the SCN but can still act independently, so SCN might not be the only endogenous pacemaker in sleep/wake cycle

  lacks population validity - research was conducted on animals and the brains of rodents are simpler and not affected by cognitive/emotional issues, so we cannot generalise to humans

  lacks ecological validity - only looks at pacemakers which cannot be separated from zeitgebers IRL as they work together
• explain the role of light in regulating the sleep/wake cycle

  without external cues like light, our internal biological clocks would continue to produce sleep/wake cycles that don't align with the external environment
• what was the aim of the study into light as an exogenous zeitgeber by campbell & murphy (1998)?


  to investigate whether light can act as an exogenous zeitgeber when detected by the skin rather than the eyes
• what was the procedure of the study into light as an exogenous zeitgeber by campbell & murphy (1998)?

  15 participants were woken at various times in the dark and a light pad was shone on the back of their knee
• what were the results of the study into light as an exogenous zeitgeber by campbell & murphy (1998)?

  participants experienced a change in their sleep/wake cycle of up to 3 hours
• what was the conclusion of the study into light as an exogenous zeitgeber by campbell & murphy (1998)?

  light is a powerful zeitgeber when detected by the skin and doesn't have to be just by the eyes to influence the SCN
• what is a negative evaluation point for research into the role of light as an exogenous zeitgeber in the sleep/wake cycle?

  lacks validity - only looks at zeitgebers which cannot be separated from pacemakers IRL as they work together

  reductionist - social cues like schedules imposed by parents or adapting to local times for sleep/meal times to get over jet lag can influence your cycle so it's reductionist to focus on light as the only external zeitgeber

  validity - irl the sleep/wake cycle is adjusted by lots of things like light and other stimuli such as alarm clocks and meal times, looking into just light effect lacks ecological validity