Endogenous pacemakers (EP) and exogenous zeitgebers work together in creating a cycle for humans, regulating differing biological rhythms. EP’s are internally caused usually by the body, and are distinct from external causes such as seasons, light, and clocks.
An example of endogenous pacemakers would be the suprachiasmatic nucleus (SCN). This is a small group of cells in the hypothalamus, directly above the optic chiasm to receive input from the eyes; rhythm can be reset by the amount of light entering the eye. It also generates its own rhythms, as a result of protein synthesis. The cells produce a protein for a period of hours, until the level inhibits further production. When the level drops below another threshold the SCN starts producing the protein again. This generates the biological rhythm. Kalat says it produces protein until you have enough. However, this is only one study and so cannot be used to generalize to everybody and therefore lacks validity.
An evaluative study would be Morgan (1995), who removed the SCN from hamsters, finding that their circadian rhythms disappeared. These rhythms could be re-established by transplanted SNC cells from foetal hamsters. Morgan also transplanted the cells from hamsters, which had been bred to have shorter cycles than normal and found that the transplanted hamsters took on the mutant rhythms. However, the validity of animal research is questionable, as the results cannot be generalized to humans. This may also be due to stress of being in a laboratory environment, and therefore lacks internal validity.
Another endogenous pacemaker is the pineal gland and melatonin. This gland contains light receptors that respond to external light via the thin layer or skull that lies above the gland. These receptors influence the activity of neurons in the gland. Melatonin acts on many organs and glands and is especially responsible for sleep/wake rhythms via the mechanism of the brainstem.
Schochat's study on the...