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Calming effects of meditation
According to a March 2006 article in the "Psychological Bulletin", EEG activity begins to slow as a result of the practice of meditation.[21] The human nervous system is composed of a parasympathetic system, which works to regulate heart rate, breathing and other involuntary motor functions, and a sympathetic system, which arouses the body, preparing it for vigorous activity. The National Institutes of Health (NIH) has written, "It is thought that some types of meditation might work by reducing activity in the sympathetic nervous system and increasing activity in the parasympathetic nervous system," or equivalently, that meditation produces a reduction in arousal and increase in relaxation.

http://en.wikipedia.org/wiki/Research_on_meditation