Following the BBC's inquiries, the UK's largest pilots' union, the British Air Line Pilots' Association, questioned 534 of its members.Click here to read the entire article on the BBC website.
Some 76% said their response times had been affected by fatigue, 72% said there had been an impact on decision-making skills and 41% said they would refuse to fly if fatigued.
But 12% said they would not feel able to refuse duties even if exhausted, while a further 33% said they would refuse - but feared disciplinary action.
A total of 63% said they are flying more hours than five years ago.
Overall, some 79% told Balpa the public should be concerned about fatigue.
Balpa chairman Mervyn Granshaw described fatigue is "the single biggest issue facing aviation".
He said: "At the moment we are not seeing it appear as accidents or incidents but we're getting closer to that point."
Click here to watch a BBC video about pilot fatigue in the UK.