The Swedish news agency TT reported 80 percent of the Swedish Airline Pilots' Association pilots said existing work rules "constitute a threat to flight safety."
"These results are alarming and we're naturally very concerned," said Mattias Kling, coordinator for work-hour issues at the association.
"We can't accept that half the pilots have fallen asleep in the cockpit."
The report said a proposal would extend the maximum number of hours in pilot shifts from the current 13 (plus up to 2 hours of overtime) to 16.
"A pilot on duty also needs to get to the airport, which often takes time," Kling said. "Then add to that 16 hours of continuous work. Is it safe to fly with that pilot?"