Endogenous pacemakers

• Endogenous pacemakers are internal biological clocks that control different biorhythms in the body
• The main endogenous pacemaker is the SCN - suprachiasmatic nucleus
• The suprachiasmatic nucleus detects light levels and is found in the hypothalamus
• The SCN is known for maintaining the sleep wake cycle
• The SCN receives info from the optic nerve about light levels and synchronises the circadian rhythm
• The SCN lies just above the optic chiasm, so even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight
• The SCN passes info on light levels to the pineal gland. At night, the pineal gland increases the production of melatonin, this is inhibited while we are awake.
• Melatonin is only produced in the dark
• Light levels detected by the optic nerve are sent to the SCN, then the pineal gland which produces melatonin, triggering the sleep wake cycle
• AO3. There is research support for endogenous pacemakers. Ralph created mutant hamsters by breeding them to have a 20 hour sleep wake cycle. He then transplanted SCN cells from the foetal tissue of the mutant hamsters into the brains of normal hamsters. The cycles of the normal hamsters became defaulted to 20 hours. This shows evidence that the SCN has a part in the sleep wake cycle. However, this study may not be applicable to humans as we are more complex and have a different biological structure to hamsters, lowering the external validity of this study.
• AO3. A weakness of endogenous pacemakers is that Folklard placed people in a lab with no light. It was found that people eventually had a sleep wake cycle of 30 hours. This supports the role of light in the sleep wake cycle, and proves the important of exogenous zeitgebers, however draws focus away from the role of endogenous pacemakers