Psychologist Shlomo Breznitz at Hebrew University, Jerusalem, had several groups of soldiers march 40 kilometers (about 25 miles), but gave each group different information.
 
He had some groups march 30 kilometers and then told them, they had to march another 10 kilometers. Another he told they had to march 60 but actually only had them march 40 kilometers. He allowed some to see distance markers and provided no clues to others how far they had walked.
 
At the end of the study Breznitz found that the stress hormone levels in the soldiers blood always reflected their estimate and not the actual distance they had marched. In other words, their bodies responded not to reality, but to what they were imaging as reality.
 
Bernie T. Siegel, Love, p 29