According to the American Psychological Association, “There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious.”
Polygraphs don’t actually detect anything meaningful except that the person is under stress – and simply being given a polygraph is sufficient to trigger the stress.
Even the creator of the machines stated that they were bunk. They’re purely a prop for interrogators.