The research carried out in Austria, Sweden, Germany and Denmark, found that four out of five pilots have to cope with fatigue while in the cockpit.
The European Cockpit Association (ECA), which represents pilots, published its chilling findings after surveys carried out by national unions, found that large numbers of pilots are half-asleep - or not even awake - while at the controls of a plane, the 'Daily Mail' reported.
Between 43 and 54 per cent of pilots surveyed in the UK, Norway and Sweden said they had already fallen asleep 'involuntarily' while flying - and a third of these pilots found their colleagues were also asleep when they woke up.
More than three out of five pilots in Sweden, Norway and Denmark reported making mistakes due to fatigue, while in Germany, this figure was four out of five.
Between 70 and 80 per cent of exhausted pilots would not declare themselves unfit to fly for fear of being stigmatised by their employers or left facing disciplinary action. More than 6,000 European pilots were surveyed for the study, between 2010 and 2012.
The ECA says the study shows that fatigue among pilots is a "common, dangerous and under-reported phenomenon in Europe".
The figures come a few weeks after the European Aviation Safety Agency (EASA) re-launched its proposals for a review of rules on flight and duty times and rest requirements for pilots.
The ECA has criticised the proposals, saying that if the EU adopts them, air passenger security will be compromised.