A poll of almost 1700 nurses in Scotland found that 54% are working more than their contracted hours each week so that they can meet demand, with 58% saying they feel under too much pressure. The survey by the Royal College of Nursing also found that 55% of respondents felt unable to provide the level of care they would like as a result of this pressure.
The RCN said its members were "running faster and faster to deliver health services that are struggling to meet demand" and lacked the resources and support required to do their job properly.
Norman Provan, associate director employment relations of RCN Scotland, said: "While for the rest of us Christmas is the season of goodwill, nurses are expected to provide goodwill all year round, with many now working extra hours for free just to keep services going.
"Indeed when you hear stories of people not being paid for extra hours because they were not agreed in advance, or people are not able to take time back in lieu because to do so would mean leaving colleagues even more short staffed, it is apparent that health services are only managing to meet demand because of nurses willing to go the extra mile for free."
The findings come as The Herald's Time for Action campaign has shown how shortages of beds and staff in some areas of the health service are impacting on patients.
Mr Provan said he hoped the findings would be a "wake-up call to all healthcare employers in Scotland as well as to the Scottish Government". He added: "While there have been welcome increases in nursing staff numbers in recent months, this comes on the back of years of cuts coupled with ever-increasing demand for services. There is no doubt among our members that many areas are seriously understaffed."
Other findings revealed that despite these pressures, nurses are still positive about their job with 65% thinking nursing is a rewarding career and 65% being enthusiastic about the job most days."
Margaret Watt, chairwoman of the Scotland Patients' Association, said she was not surprised by the findings. She said: "It's gone on too long. Our nurses are overworked, stressed, and they can't do their best. The vast majority of them are doing a fantastic job, but it's no wonder people feel like they can't do their best."
Scottish Liberal Democrat health spokesman Jim Hume MSP said: "After slashing nearly 2000 nursing posts between September 2009 and December 2012 the SNP has left holes in the NHS workforce and this is clearly taking its toll on our hard-working nurses."
A spokesman for the Scottish Government said nurses "were at the heart of Scotland's NHS, which is why we are ensuring they get a pay rise this year, and why we have rejected moves by the UK Government to remove their right to pay progression."
He added: "Nobody in the NHS should be working for nothing. The Cabinet Secretary has asked all boards for their assurance that all staff are receiving the full reimbursement."