"It has been hard to show any clear effects on health," Carina Bildt at the National Institute for Working Life told Swedish Radio's Ekot programme.
"That could either be because there aren't any, or because there hasn't been a decent evaluation of the benefits."
A six hour work day is a key policy of Sweden's new feminist party, the Feminist Initiative, which argues that it will make working life more accessible to women with children. The Left Party has also shown support for cutting working hours.
The idea is not a new one in Sweden, and a number of district councils have experimented with shorter working days at the same salary. But for various reasons they have given up on the concept.
According to Ekot, the introduction of a six hour day in the child care sector was a failure because the cost of hiring temporary workers became too expensive. And at a hospital in Stockholm a test in one department was cancelled because staff in other departments became resentful.
Kiruna was the flagship of the policy, but Hans Sedwell, the vice chairman of the council, said that it was simply too expensive and complicated.
"It has been costly for the district and has proven that an organisation does not work with two different agreements on working hours," he said.
Carina Bildt said that evidence from France showed that a shorter working day could actually be bad news for workers' health.
"People have seen there that the intensity of the job increases significantly, with negative effects on health as a consequence. It has certainly helped to improve productivity, but sickness has also increased," she said.