A new regulation has come into force this month which demands that all heads are given the opportunity to develop a satisfactory work-life balance.
The rule insists it is governors' responsibility to ensure their school leader has sufficient free time to pursue "personal interests".
But heads' leaders have warned that the rule change is completely pointless unless the burden of bureaucracy forcing them to work hugely long hours is tackled.
Previously, governors were asked to have "due regard" for work-life balance, but this is the first time it has been laid out in such black and white terms.
The new rule says: "The governing body must have regard to the desirability of the headteacher being able to achieve a satisfactory balance between the time spent discharging the professional duties of a headteacher and the time spent by the headteacher pursuing personal interests outside work."
It is buried within the School Staffing Regulations 2009, which came into force earlier this month.
But Mick Brookes, general secretary of teaching union the NAHT, is far from convinced it will make any difference.
"It's absolutely right to set this duty out in law, but there needs to be some cessation in the amount of bureaucracy and administration - much of it spawned by the (Department for Children, Schools and Families) and local authorities," he said. "What we've been saying about heads' workload has been completely ignored. Instead, they've had to follow up wacky government ideas that are proving an increasing burden.
"What's needed is a serious look at this on a case-by-case basis so we can tackle this culture. School leaders are being harassed and bullied by those who want to hold them to account."
Fellow heads' union the Association of School and College Leaders is also sceptical about the impact of the new legislation. John Dunford, the union's general secretary, thinks the new law is as vague as the previous one.
"It's not about the unwillingness of governing bodies to do anything, but they are powerless to do anything when there is this constant stream of legislation," he said.
"As long as there's this level of change in the system, nothing like this will have any impact."
BEAST OF A BURDEN
Fiona Hammans, head of Banbury School in Oxfordshire, puts in about 70 hours a week - and that's not counting the hours lying awake worrying, she says. Workforce agreements have reduced teacher absence and supply staff costs at Banbury, but Dr Hammans has seen her responsibilities soar. She doesn't think the legislation will allow her to spend more time on personal interests unless constant government checks on schools ease.
"We are trying to get the best possible outcome for pupils, but the burden seems to be getting bigger and bigger," she said. "Everything has got to be perfect, it's harder and harder to prioritise. But heads have to check everything - it's our job."