When some choirs sing, the individuals' heart rates quickly synchronize. Researchers from the University of Gothenburg in Sweden monitored the pulse of high school choir singers and also found that when the choir began to sing, their unified voices caused their heart rates to slow.
"When you sing the phrases, it is a form of guided breathing," project leader and musicologist Bjorn Vickhoff told National Public Radio. "You exhale on the phrases and breathe in between the phrases. When you exhale, the heart slows down… The members of the choir are synchronizing externally with the melody and the rhythm, and now we see it has an internal counterpart."