endogenous pacemakers and exogenous zeitgebers

• endogenous pacemakers are internal body clocks that keep biological processes to time
• exogenous zeitgebers are external cues that entrain internal body clocks, altering the body clocks to match the environment
• an example of an endogenous pacemaker is the suprachiasmatic nucleus (SCN)
• an example of an exogenous zeitgeber is light and social cues (e.g. meal time)
• the suprachiasmatic nucleus is one of the primary endogenous pacemakers and it is influential in the sleep/wake cycle
• the suprachiasmatic nucleus is located above the optic chiasm, the point on the optic nerves where they cross
• the suprachiasmatic nucleus receives information about light levels and the SCN then sends signals to the pineal gland, which produces melotonin to induce sleep
• ralph (1990) - evidence that the SCN is an endogenous pacemaker

  hamsters with tau mutation have circadian rhythms of 20 hours, compared to 24 hours for normal hamsters

  tau hamsters were given SCN from normal hamsters, changing their circadian rhythm to 24 hours

  normal hamsters were given SCN from tau hamsters, changing their circadian rhythm to 20 hours
• light is a key zeitgeber in humans, it can reset the body’s main endogenous pacemaker - the SCN, impacting the sleep/wake cycle and keeping it on a 24 hour cycle
• social cues, such as meal times, also act as zeitgebers and humans can compensate for the lack of natural light by instead using social cues