Teachers have a guaranteed half a day a week away from children for planning and assessment, but the heads' contract is not specific. As a result, most heads get fewer than five hours away from the competing demands of staff and officials, pupils and parents.
Some schools are providing time for their heads: one board of governors insists its headteacher works from home one day a month so that he can focus on the big picture. But a third of heads receive no dedicated time at all.
The findings have prompted the National Association of Head Teachers, which commissioned the survey of 985 school leaders, to push for legal protection for busy heads at their conference in Bournemouth beginning today.
Mick Brookes, the association's general secretary, said the situation would only get worse if the Government did not provide schools with enough resources to allow them to delegate inappropriate work.
Official figures last year identified gradually decreasing workloads for classroom teachers, but primary heads' work increasing to 53.5 hours a week, and secondary heads' to 65 hours.
Some of the heavy workloads are borne by superheads such as Ninestiles'
chief executive Sir Dexter Hutt, who estimates he works 75 hours a week, but often it is the hard-working heads in small isolated schools who really bear the brunt.
Paul Sunners, head of the 46-pupil Nyland special school in Swindon, is one of those behind the motion to the NAHT conference in Bournemouth.
He compared his school to working in a hospital intensive care ward. He said: "Some of the children are deeply troubled, so when they are in school I have to be very accessible and hands on.
"I also have to do everything from minibus runs to sorting out blocked toilets and picking up dog shit off the playing field."
The story of his 65-hour week is a familiar one for heads. He starts work at around 6.30am, seven days a week, to get in a clear hour or two in the morning before the interruptions begin.
He said: "I celebrate dawn. When the sun rises, it's glorious. But the odd lie-in might not go amiss. That's the cost of serving people; that's what drives people to work so hard within the profession."
Dedicated headship time would allow him to shut the office door and spend time working on the school development plan, or to devise a path through such government initiatives as performance management.
A National Union of Teachers' survey this week shows heads spend a third of their time on bureaucracy and paperwork, nearly five times what they believe should be the case. This means they are unable to devote as much time to strategic leadership and interaction with children as they believe they should.
Two out of three heads say they have suffered stress-related illnesses, according to a Keele University survey published today.
They cited mistakes including overdrawing the school budget, receiving an "inadequate" inspection rating for an under-prepared lesson, and crashing their cars late at night after governors' meetings.