“When you sing the phrases, it is a form of guided breathing,” says musicologist of the Sahlgrenska Academy who led the project. “You exhale on the phrases and breathe in between the phrases. When you exhale, the heart slows down.”

But what really struck him was that it took almost no time at all for the singers’ heart rates to become synchronized. The readout from the pulse monitors starts as a jumble of jagged lines, but quickly becomes a series of uniform peaks. The heart rates fall into a shared rhythm guided by the song’s tempo.

“The members of the choir are synchronizing externally with the melody and the rhythm, and now we see it has an internal counterpart,” Vickhoff says.