Exogenous Zeitgebers

• Zeitgebers - time givers
• Exogenous zeitgebers - external factors that affect biological clocks
• Two main exogenous zeitgebers:

  light levels

  social cues
• Exogenous zeitgebers light levels - indicate sleeping patterns, specifically the sleep wake cycle
• Exogenous zeitgebers social cues - food schedule can indicate when you should or shouldn’t sleep

  social interactions such as loud and lively environments can have an effect on your sleep wake cycle
• Melanospin - a protein found in the rods and cones of your eyes that detect light

  Needed for the detection of external light
• AO3. There is evidence for exogenous zeitgebers. Most blind people can still detect light levels as they still have melanospin in their eyes which allows them to detect light levels without seeing. This shows evidence for light levels and melanospin being an important factor in the sleep wake cycle.
• AO3. A limitation to exogenous zeitgebers is that Siffre stayed in a cave for 60 days with no exogenous zeitgebers such as light levels or social cues and found that his sleep wake cycle remained around 24 and a half hours. This means that his sleep wake cycle remained the same, even though he had no external cues. However, as Siffre believed that he had been in the cave for a shorter amount of time as he felt the days were longer