endogenous pacemakers

• Endogenous pacemakers are biological “clocks” that are generated from within organisms.
• In mammals the main pacemaker is a tiny cluster of nerve cells called the Suprachiasmastic nucleus (SCN) located in the hypothalamus just above where the optic nerves from each eye cross over. The Suprachiasmastic nucleus (SCN) obtains information via these optic nerves regarding light (an exogenous zeitgeber) and this occurs even when our eyes
 are closed as light penetrates the eyelids. This produces a circadian rhythm which is reset by light entering the eyes
• The SCN receives information on light and sends signals to the pineal gland which then regulates the production of melatonin which induces sleep and inhibits brain mechanisms promoting wakefulness. Light inhibits melatonin production with the SCN indirectly involved in this process and this demonstrates how endogenous pacemakers interact with exogenous zeitgebers.