These findings suggest that music will enhance activity of the parasympathetic nervous system. When catecholamines (stress hormones) levels are lowered and we are not in an emergency phase we feel more relaxed.
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This effect could be connected with the repetative behaviour we engage in when we are anxious (such as pacing or tapping our foot). This helps relax us.
Music could simply be a complex learnt behaviour that we have discoved calms us down.
As an alternative to thses findings:
LF/HF, considered to be an indication of sympathetic nerve activity increased significantly during listening to hard rock music.
— | dxlive03
Indicating that some music can have the opposite effect. One could suppose that the beat of the music interacts with your nervous system in a complementary way; calming you down or psyching you up.
Veiwing this as a tool (and thinking about how good people are at using more and more sophisticated tools) we could suggest that music has evolved to act as a tool for mood modification.