Stress is driving headteachers to take "astronomical" amounts of sick leave, research to be released later this month is expected to reveal.
The survey by the National Association of Head Teachers comes as growing numbers of school leaders quit early amid complaints about the pace and scale of change being forced on schools.
Liz Howe, head of 85-pupil Great Bowden Church of England primary, in Market Harborough, Leicestershire, is retiring two years early. "I need a life," she said.
"I have coped for many years, but heads are now not trusted. Last year, I had 12 weeks off with work-related stress. This summer I spent four weeks of my summer holiday in school. I spend most weekends at school."
Miss Howe, 57, reflected the anger among delegates at the NAHT's primary conference in Oxford last weekend at what they see as an unending stream of ill- thought-out initiatives.
Heads say there is still too little funding for the half-day a week they have to give teachers for planning, preparation and assessment and not enough potential staff to run extended schools.
They are fed up too with Ofsted's self-evaluation forms and having to restructure their workforces, which could lead to pay cuts for teachers.
Schools have to agree plans to replace management responsibilities payments with those for teaching and learning responsibilities by Christmas and have until the end of 2008 to implement the new scheme.
Roger Jackson, head of Huntington primary, York, said: "Heads know restructuring is going to mean some teachers will suffer financially."
He is to retire at Easter when he will be 57. He said: "It is the right time for me to leave. As far ahead as you look there are more things coming, such as the national curriculum being reassessed. There is never any let-up and there hasn't been in the past 10 years."
Angeles Walford, head of the Priory C of E primary in Wimbledon, south London, said: "It's a living nightmare being a primary head. You think you've just got to the top of the pile and another tranche of initiatives comes along."
Another London head, who asked not to be named, said she expected to leave by the time she was 50.
She said: "Headteachers are feeling demoralised. Heads are busting a gut to raise morale and make sure teachers are happy, but who is looking after us?"
Mick Brookes, general secretary of the NAHT, said heads should refuse to be pushed around any longer.
He said: "We should seize back the agenda. Let's not be bullied into any new initiatives without thinking it out properly in terms of capacity, funding and time.
"It is not the Government which raises standards, it is heads and teachers.
And we should take credit."
Research for the NAHT and Secondary Heads Association by John Howson, of Education Data Surveys, found that one in five schools which advertised for a head last year failed to appoint.
Professor Howson said: "I think for many people the problems of restructuring the school may be the last straw. They are just feeling they have had enough."
Leadership 28Friday magazine 11