ENDOGENOUS PACEMAKERS / EXOGENOUS ZEITGEBERS

• CIRCADIAN RHYTHM 


  The sleep wake cycle (8 hours-16 hours) occurs approximately every 24 hours
• CIRCADIAN RHYTHM 


  affected by internal (endogenous) factors e.g our internal body clock and external (exogenous) factors e.g light.
•  The main endogenous pacemaker for the circadian rhythm is the suprachiasmatic nucleus (SCN), part of the hypothalamus, which acts as an internal body clock to keep the body on an approximate 24 hour sleep-waking cycle. It is sensitive to light and regulates the pineal gland, which secretes melatonin - a hormone which encourages sleep. When there is less light, more melatonin is produced. When there is more light, levels are reduced and waking occurs.
• (-) ENDO / EXO

  VETTER ET AL =

  investigated light's importance in regulating the sleep-wake and activity-rest patterns. 1 group of remained in normal 'warm' artificial light for 5 weeks, while another group experienced artificial 'blue-enriched' light similar to daylight. The 1st group synchronised their circadian rhythms each day with the natural light of dawn. but, 2nd group synchronised their rhythms to office hours. This indicates that light is the dominant zeitgeber for the SCN and that its effectiveness depends on its spectral composition.
• (-) ENDO / EXO

  FOLKARD =

  had a volunteer spend 25 days in the controlled environment of a laboratory. The participant had no access to daylight or other zeitgebers that might have reset the SCN. At the end of the 25 days her core temperature rhythm was still at 24 hours. However, her sleep-wake cycle had extended to 30 hours, with periods of sleep as long as 16 hours being recorded.
• (+) ENDO / EXO

  ASCHO ET AL =

   showed that individuals are likely to respond to social zeitgebers in the absence of natural zeitgebers such as natural light, sleeping and waking at set times of the day.
• (+) ENDO / EXO

  MORGAN =

  bred hamsters so that they'd have abnormal circadian rhythms of 20 hrs instead of 24 hrs. SCN neurons from these abnormal hamsters were then transplanted into the brains of normal hamsters. normal then displayed the same abnormal circadian rhythms of 20 hrs, showing that the SCN that was transplanted had imposed its pattern onto the recipients' brains. But in the reverse experiment, normal scn to abnormal, the recipient hamster's circadian rhythms changed to 24 hrs. = endogenous pacemakers play an important role in maintaining circadian rhythms