The earliest music, that of the most primitive cultures, is often limited to percussion, the beat. The beat is all-important, conveying a message all on its own. Part of that message is physiological. It's no surprise that the loudest, most steady drum in primitive music is called the "heartbeat drum." Early additions to drum music served to emphasize the rhythm. Native American music, for instance, adds "vocables" which are nonspecific vocalizations, or chants, which serve mostly to accent the rhythm. The beat may be slow in a courtship dance or fast in a celebration dance and rising in intensity to prepare warriors for battle. Primitive people knew that rhythm could affect how people felt and they used it to intensify feelings of love, happiness, anger, etc.
Newborns like nothing better than to be held up near their mother's hearts. The sound of a rhythmic heartbeat calms newborns. A calm mother has a slow, steady heart rate and so does a calm baby, who takes its cue from its mother. Lullabies, which, like a calm heartbeat, are slow and rhythmic, have been used for centuries to calm a fussy newborn and "lull" them to sleep. Recording companies have capitalized on this idea by recording maternal heartbeats, even some including womb sounds, and enhancing them with a correspondingly gentle and rhythmic musical track.
Driving heart rates with music has gone high tech. Studies are ongoing using MRI, fMRI, MEG, SQUIDS, PET scans and EEGs to assess a person's brain function while listening to music. Enough evidence is mounting that scientists are looking at using music to aid in physical healing, helping those with neurological impairments and even reducing pain. Music lowers the heart rate and distress levels of heart patients. Calm, rhythmic music played in surgical theaters has a double benefit: it calms patients and surgeons alike.
Studies are not definitive on the effect of music on heart rate. Studies at two hospitals, Bryan Memorial in Nebraska and St. Mary's Hospital in Wisconsin, found that, in patients recovering from surgery, music reduced heart rates and helped regulate blood pressure and respiration. Another study, however, found there was no affect on heart rate when rhythmically homogenous music (music with rhythms that did not change) was played for 30 minutes or more. They did find significant change in blood pressure and found that patients' breathing slowed down as well.
Rhythm is as natural as walking and breathing. Perhaps the reason hearing music affects heart beats is because we are wired to take cues about the environment from each other. It starts in the womb: when the mother's heartbeat increases, so does that of the fetus. When people perceive others around them breathing faster, their breath increases and so does their heart rate. In a group, excitement is contagious, and so is calmness.