... Morgan also transplanted the cells from hamsters who 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 generalised to humans. 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 involvement of melatonin in the sleep waking cycle is some of the strongest evidence for this area of research. Their six male participants spent 29 hours between 7am one day to noon the following day in the sleep lab. In this time they ...

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