Two pilots working for an airline in the United Kingdom (UK) were found to have fallen asleep while operating a flight mid-air.
Both men were alone in the cockpit when their co-pilots left the flight deck, the Civil Aviation Authority revealed.
The authorities refused to name the airlines, claiming it would breach confidentiality. But the British Airlines Pilots Association say the problem is common.
It revealed that 43 per cent of members polled admitted sleeping in the cockpit.
In one of the incidents uncovered after a Freedom of Information request, the captain had left the cockpit to use the toilet but had to use a code to get back in the cockpit because he found the pilot "slumped over the controls", The Sun reported.
Another pilot also found himself unable to get back in the cockpit and used the entry code. His first officer had to be shaken awake.
A third pilot also fell asleep, while his plane was on the ground.
Rob Hunter, the union's head of safety, told the Transport Select Committee earlier this year that pilots feared being disciplined if they complained of being tired.
"We commonly receive letters that deal with cases where pilots feel that the process that they then get embroiled in is more fatiguing that the duty itself. It becomes a better option to put up with a bit of fatigue rather than report it," Hunter was quoted by the Daily Telegraph as saying.