endogenous pacemakers and exogenous zeitgebers

• Endo/exogenous

  Endogenous means within the organism, and pacemakers allow us to keep pace with changing cycles. Exogenous means outside the organism and zeitgeber comes from the German 'time giver'.
• The SupraChiasmatic Nucleus (SCN)

  A cluster of nerve cells. This acts as a master clock - links to other brain regions to control sleep/arousal and other rhythms. Neurons within the SCN synchronise, so neurons receive time co-ordinated signals. The SCN resets when external light levels change.
• Pineal Gland

  SCN signals the gland to produce melatonin at night and stop in the morning. Melatonin inhibits brain mechanisms that promote wakefulness.
• Light

  Lights resets the SCN, keeping it on a 24-hour cycle. A protein called melanopsin in a few retinal cells gauge the overall brightness and carries signals to the SCN
• Social Cues

  Aschoff et al (1971): individuals are able to compensate for a lack of natural light by responding to social zeitgebers. Klein and Wegmann (1974): circadian rhythms of air travelers adjusted more quickly when they went outside after landing.
• The role of the SCN

  Morgan (1995): bred hamsters with 20 hour circadian rhythms. The SCN of these hamsters was transplanted into normal hamsters and the normal hamster's SCNs were transported into the specially bred ones. Each set of hamsters automatically adjusted to the new rhythm
• Separate rhythms

  Sometimes the rhythms become out of sync. Folkard (1994) studied a student who volunteered to spend 25 days in a controlled environment with no external zeitgebers. While her sleep/wake cycle was 30 hours,, her body temperature still had a 24 hour rhythm.
• The SCN and Zeitgebers

  Skene and Arend't (2007): the vast majority of blind people who have light perception have normally entrained circadian rhythms, whereas those without this perception do not have this.
• Application

  Burgess (2003): exposure to bright light before an east-west flight decreased the time needed to adjust to local time. Participants were exposed to either a bright light, an intermittent light or a dim light before flight. Those exposed to bright light shifted their rhythm, by 2.1 hours.
• Artificial light

  Veter et al (2011): 2 groups of volunteer participants studied over 5 weeks. One group had 'warm' light and the other had 'blue enriched' light. Those with warm light synchronised their rhythms with the natural light of dawn, which over the course of the study, advanced by 42 minutes, This change did not occur in those exposed to blue light.
• beyond the master clock - endogenous pacemaker

  P: limitation - may obscure other body clocks

  E: peripheral oscillators found in organs such as the lungs that are influenced by the SCN but also act independently

  E: changing feeding patterns in mice could change the circadian rhythm of cells in the liver for up to 12 hours

  L: therefore there are other complex influences on the sleep/wake cycle
• interactionist system - endogenous pacemakers

  P: limitation - cannot be studied alone

  E: Siffre used an artificial light that may have reset his biological clock every time he put the lamp on

  E: in everyday life, pacemakers and zeitgebers react so it makes little sense to separate them for research

  L: therefore researchers reduce the validity of their research by studying internal pacemakers alone
• environmental observations - exogenous zeitgebers

  P: limitation - do not have the same effect in different environments

  E: the experience of people who experience little darkness in the summer and little light in the winter is different to the usual

  E: e.g. those who live in the Artic Circle have similar sleep patterns all year round despite spending 6 months in darkness

  L: therefore sleep/wake cycle mainly controlled by endogenous pacemakers that override environmental cues
• case study evidence - exogenous zeitgebers

  P: limitation - evidence challenges role of exogenous zeitgebers

  E: a study of young man, blind from birth, who had an abnormal circadian rhythm of 24.9 hours

  E: despite exposure to social cues such as normal mealtimes, his sleep/wake cycle could not be adjusted

  L: therefore social cues not effective in resetting the biological rhythm