The brain exerts a restraining influence over the heartbeat via the vagus nerves. This is known as cardiac vagal ‘tone’. It is strong when we are healthy but weak when we are sick. It has a positive influence on the health of the heart and perhaps the body generally. Vagal tone comes from the brain but, despite over 150 years’ research, nobody knows exactly where. Using a novel technique, we can track down its origins. This important basic knowledge may lead to ways to improve health.