Geoff Lucas, general secretary of the Headmasters' and Headmistresses'
Conference, said such investment reaped rewards, with hundreds of heads opting to stay longer in their posts.
Research by the HMC, which represents 240 fee-paying schools, including Eton, Rougemont in Newport, and Llandovery college, Carmarthenshire, shows that a fifth of heads had been given a sabbatical, normally of a term.
Mr Lucas said the emphasis on "nurturing and feeding" private school heads had a knock-on effect, with three-quarters staying at the same school for five years or more and two-fifths remaining in the post for more than a decade.
In recent years, demands for sabbaticals have been repeated by the National Association of Head Teachers and the Association of School and College Leaders as a means of combating stress.
John Dunford, general secretary of the ASCL, said: "A very strong incentive to keep good headteachers would be a period of study leave after five years. It would presumably provide heads with some obligation to stay at the school a bit longer and the work they undertake could have a huge benefit to the school. It is very difficult, as a headteacher, to find the time to do any strategic thinking."
The HMC surveyed almost 200 members and found 47 per cent had been head at their current school for five years or more. Among heads with more than five years' experience, 18 per cent had had paid study leave.
Mr Lucas, writing in The TES Cymru today, said that five years was the minimum time heads needed to "bring about meaningful change" in a school.
He said paid study leave should be considered as an incentive to keep heads.