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Researchers have known for years that animals have a ’clock’somewhere in their brains a control-centre that regulates circadian（生物节奏的）rhythms.Circadian rhythms are the body’s daily biological cycles，controlling such activities as sleep and wakefulness.In November 1988，neuroscientists at the University of Virginia in Charlotteville reported that the supra chasmatic nucleus（SCN），a small cluster（束）of cells in the middle of the brain that neuroscientists have long suspected of playing a role in daily rhythms，is the long-sought biological clock.
The investigators performed a series of tissue-transplant experiments with hamsters （仓鼠）.Some of the animals were normal hamsters，which have a circadian cycle of 24hours；the others were mutant（变异型）hamsters whose cycles were only 20or 22hours long.
The scientists first removed the SCN from the brans of the hamsters in both groups.They then implanted an SCN from a mutant hamster into the brain of each normal animal，and vice versa.When the hamsters had recovered from their operations，the circadian rhythms of the two groups were reversed.The normal hamsters had a daily cycle of 20to 22hours，and the mutants were on a 24-hour cycle.
The identification of the internal clock is an important stepping-stone in understanding the basic，automatic mechanisms of brain function.It may also help in finding ways to relieve human sleep disorders.