Endogenous Pacemakers and Exogenous Zeitgebers

• Endogenous pacemakers are internal body clocks that regulate many biological rhythms such as the sleep/wake cycle
• Exogenous zeitgebers are external cues that entrain out biological rhythms such as the influence of light on the sleep/wake cycle
• The Superchiasmatic nucleus (SCN) is located in the hypothalamus of each hemisphere of the brain
• The superchiasmatic nucleus is the endogenous pacemaker for many biological rhythms such as the sleep/wake cycle
• the superchiasmatic nucleus is connected to the optic chiasm to receive information about light changes from the left and right visual cortex
• The superchiasmatic nucleus also impacts the production of melatonin and is therefore involved in the onset of Seasonal Affective Disorder a infradian rhythm
• Light is an exogenous zeitgeber that controls the sleep wake cycle and can reset the superchiasmatic nucleus
• Social cues are an exogenous zeitgeber that control many biological rhythms such as meal times
• DeCoursey (2000) destroyed the superchiasmatic nucleus in 30 chipmunks before placing them into a natural habitat for 80 days to be observed
• DeCoursey (2000) found that the destruction of the superchiasmatic nucleus destroyed the chipmunks sleep/wake cycles with most being killed by predators as they were out at night when predators were active
• Ralph (1990) created mutant hamsters with a 20 hour sleep/wake cycle and found that when the superchiasmatic nucleus was put into normal hamsters their sleep wake cycle followed this as well
• Campbell and Murphey (1998) found that light shined on the back of participants knees disrupted 15 participants sleep wake cycles showing that it is a powerful exogenous zeitgeber that can be interpreted even indirectly