A BBC undercover reporter who got a job at Amazon's Swansea warehouse secretly filmed night shifts involving up to 11 miles of walking, with workers expected to collect orders every 33 seconds.
Michael Marmot, one of Britain's leading experts on stress at work, reviewed the footage and said the working conditions at the warehouse are "all the bad stuff at once".
He said: "The characteristics of this type of job, the evidence shows increased risk of mental illness and physical illness."
The development came as Amazon employed 15,000 extra staff to cater for Christmas orders. Amazon said the safety of its workers was its "number one priority".
BBC reporter Adam Littler, 23, got the agency job at Amazon's warehouse. He was employed as a "picker", collecting orders from 800,000 sq ft of storage.
A handset told him what to collect and put on his trolley. It allotted a set number of seconds to find each product and counted down. If he made a mistake, the scanner beeped.
"We are machines, we are robots, we plug our scanner in, we're holding it, but we might as well be plugging it into ourselves," Littler said.
Marmot said, "There are always going to be menial jobs, but we can make them better or worse. And it seems to me the demands of efficiency at the cost of individual's health and wellbeing - it's got to be balanced."
Amazon said official safety inspections had not raised any concerns and an independent expert had advised that the picking job is "similar to jobs in many other industries and does not increase the risk of mental and physical illness".
The scanner tracked Littler's picking rate and sent his performance to managers. If it was too low, he was told he could face disciplinary action.
Experts told BBC that Amazon's ten-and-a-half-hour night shifts could breach working time regulations because of long hours and strenuous nature of the work.