endogenous and exogenous

• Endogenous pacemakers are the products of inherited genetic mechanisms and allow us to keep pace with changing environments. In mammals, the main endogenous pacemaker is a cluster of nerve cells called the suprachiasmatic nucleus (SCN), which lies in the hypothalamus. The SCN has an important role is generating the circadian rhythm, it acts as the ‘master clock’ with links to other brain regions that control sleep and arousal.
• Neurons within the SCN synchronise with each other so their target neurons in sites elsewhere so the body receives correctly time coordinated signals. These peripheral clocks can maintain a circadian rhythm, but not for very long so they must be controlled by the SCN. SCN only needs resetting when external light levels change. This information comes from the optic nerve. The SCN also regulated the production and secretion of melatonin in the pineal gland.
• . The pineal gland receives the signals and increases secretion at night, and reduces it in the morning. The sensitivity of the pineal gland and SCN to light, means that their activity must be synchronised with the light-dark rhythm of the outside world. Exogenous zeitgebers are from outside of the organism. Light resets the internal biological clock, keeping it on a 24 hour cycle. Rods and cones in the retina of the eye detect light to form visual images.
• . A protein called melanopsin is sensitive to natural light and critical in this system. Social cues can also be used as exogenous zeitgebers. Wegmann found that circadian rhythms of air travellers adjusted more quickly if they went out more at their destination. This was said to be due to the exposure of the social cues in their destination.
• Morgan (1995) bred a strain of hamsters so they had abnormal circadian rhythms of 20 hours rather than 24 hours. SCN neurons from the abnormal hamsters were transplanted into normal hamsters. the normal hamsters then displayed 20 hour circadian rhythm. the SCN of normal hamsters was put in abnormal and they then showed 24 circadian rhythm. 

  This confirms the importance of the SCN in setting circadian rhythms.
• Its argued that artificial light at night results in a disruption of the circadian system. For example research has shown that teenagers spend increasing amounts of time on electronic media at night. the LED bulbs of these devices are enriched with a blue light component very active on the circadian clock which leads to suppression of melatonin secretion and circadian disruption. as a result the sleep becomes irregular and shortened. In the long run this combination of sleep deprivation and disruption is detrimental to health as shown by studies which found increased rates of depression.
•  The important role played by melanopsin in setting the circadian rhythm is demonstrated by blind people.

  Some are still able to reliably entrain their circadian rhythm in response to light despite a total lack of image-forming visual perception (i.e. non-functioning rods and cones). Its estimated that the vast majority of blind who still have some light perception have normally entrained circadian rhythms.

  This suggests that the pathway from retinal cells containing melanopsin to the SCN is still intact.
• The importance of light in the regulation of the sleep-wake and activity-rest patterns has been shown. One group remained in normal 'warm' artificial light over the 5 weeks while the other group had artificial (blue) lightt. All pts kept a sleep log and devices measured their movement over each 24-hour period. Pts working under the warmer light synchronised circadian rhythms with natural light. Over the study, sunrise advanced by 42 mins. The pts exposed to blue-enriched light did not show the same 42-minute adjustment. The results confirm that light is the dominant zeitgeber for the SCN.