Undercover filmmakers pretending to be workers in a factory making Apple products in China had to work up to 16 hours a day and as many as 18 days in a row, according to an investigation by BBC Panorama.
The programme, broadcast on Thursday, said that pledges by the US tech giant to ensure decent working standards were often broken.
Workers were filmed as they fell asleep during 12-hour shifts at the Pegatron factories near Shanghai.
However Apple disputed the programmes findings but said it was working with its suppliers to improve working conditions.
One undercover reporter, who worked a shift lasting 16 hours, said: “Every time I got back to the dormitories, I wouldn't want to move. Even if I was hungry I wouldn't want to get up to eat. I just wanted to lie down and rest. I was unable to sleep at night because of the stress.”
None of the reporters were given a choice about whether to do overtime even though it is supposed to be voluntary.
The BBC said Apple had declined to be interviewed for the programme.
However the firm said in a statement: “We are aware of no other company doing as much as Apple to ensure fair and safe working conditions.
“We work with suppliers to address shortfalls, and we see continuous and significant improvement, but we know our work is never done.”
Apple said that workers would commonly sleep during breaks, but added it would look into the claim that they were falling asleep when they were at work. It added that staff at Pegatron worked on average for 55 hours a week.
Pegatron said it was looking into Panorama’s findings.
“Worker safety and well-being are our top priorities. We set very high standards, conduct rigorous training for managers and workers, and have external auditors regularly visiting our facilities to find areas for improvement,” it said.