Four-fifths of teachers have sacrificed a night's sleep in the last six months to get through school work, according to a survey.
It reveals that many teachers are spending time at the weekend and during holidays trying to catch up with marking, lesson planning and admin.
The poll, by tesconnect.com, suggests the idea that teachers finish work at 3pm and take long holidays is a myth, with the majority working more than 56 hours a week on average.
Around 80% of teachers said they sacrificed a night's sleep to get through a backlog of work, with 41.7% giving up a night in the last month.
More than three-quarters (78%) of around 1,600 people surveyed agreed that during term-time they spend every Sunday afternoon or evening working on preparing lessons, while almost two-thirds (64%) said the "hidden" hours they spend on school-related work, for example at home, has affected their health.
Around four-fifths (81%) said that these hidden hours affected their personal life. More than half (54.5%) of teachers said they work more than 56 hours a week on average. A quarter (24.1%) said their average working week was between 56 and 60 hours, 15.9% said 61 to 65 hours, and 14.5% said it was 66 hours or more.
Around one in four (24.6%) said they spend between five and eight hours a week on school-related work during the holidays, with 10.1% saying they spend less than four hours on work during this time, and two-thirds (65.4%) saying they spend nine or more hours a week on it.
Louise Rogers, chief executive of TSL Education, the parent company of TES Connect, said: "It comes as no surprise to us that teachers are amongst the hardest working profession. Our site is busiest on Sunday afternoons.
"When most other professions are taking a rest, teachers are busy preparing for the week ahead."
Christine Blower, general secretary of the National Union of Teachers (NUT), said the survey will be of no surprise to teachers: "Until the high-stakes nature of league tables are removed and teachers are given more trust in the workplace, then working hours are unlikely to reduce."