The study, which included responses from 625 pilots working at all the different airlines in Sweden, also found that 70 percent of them admitted to having made mistakes caused by tiredness, the Dagens Nyheter (DN) newspaper reported.
In addition, 80 percent of Swedish pilots reported that existing work hour rules "constitute a threat to flight safety".
According to current EU regulations, a pilot is allowed to work shifts as lengthy as 13 hours, sometimes with two additional hours of overtime.
"These results are alarming and we're naturally very concerned," said Mattias Kling, coordinator for work hour issues at the SPF.
"We can't accept that half the pilots have fallen asleep in the cockpit."
And despite the alarming study, there's a proposition to extend allowed the length of pilot shifts to 16 hours.
“A pilot on duty also needs to get to the airport which often takes time. Then add to that 16 hours of continuous work. Is it safe to fly with that pilot?” Kling told TT.
Kling doesn't want to comment on the risk this poses to passengers, but he's not happy with current regulations, and pilots now demand that these be based on scientific findings.
“If you don't have scientific facts you need to practice the precautionary principle,” Kling said.
“You have to be certain what the rules you impose mean. That's the message we want to get across.”