Abstract
Eureka moments are so rare in science that when you have one, you don't forget it in a hurry. My colleagues and I at Napier University in Edinburgh were fortunate enough to have one such moment a few years ago when we were studying the signals from a medical device called a pulse oximeter. Widely used in hospitals to measure the percentage of blood haemoglobin that is saturated with oxygen, the device also provides an accurate measure of a patient's heart rate.