Babies who die from SIDS are low in a brain chemical that controls heads and neck movement, meaning they can't shift out of dangerous positions.

Babies who die of SIDS typically lack a brain chemical that helps control head and neck movement, leaving them unable to move out of life-threatening positions, a breakthrough Australian study has shown.

The findings could soon lead to screening tests to identify babies most at risk of sudden infant death Syndrome (SIDS).

University of Adelaide Professor Roger Byard says a crucial part of the SIDS puzzle has now been solved.

He and his team tested brain samples from babies who died of SIDS and found that in the vast majority of cases they had much lower levels of substance P.

Substance P is a chemical in the back of the brain that controls a range of functions, including the response to low-oxygen situations such as the ones babies can get into when they roll onto their tummies.

"What we found in these SIDS babies is that they had significantly lower levels of substance P in areas that are related to the movement of the head and neck," Prof Byard told AAP on Wednesday.

"For 25 years we've been saying, why do SIDS babies not get out of a dangerous situation when they are face down? Why don't they just lift their heads up and cry and thrash around to get their parents' attention? Well, they just don't move as well as normal babies do."

Prof Byard said the vast majority of brain samples he and his colleagues, Professor Robert Vink and Dr Fiona Bright, examined were short of substance P.

That was particularly true for boys, who account for twice as many SIDS deaths as girls.

"We really have opened a doorway into a whole new area of SIDS research. What we need to do now is work out why the levels are low and is there some way of testing these kids?"

Prof Byard also said a landmark US study, later replicated by his team using Australian samples, had revealed reduced levels of serotonin in the brain stems of SIDS babies.

"Essentially all these chemicals work together, serotonin and substance P, and certain levels are required to stimulate a response," he says.

The Substance P study was carried out in collaboration with Harvard Medical School and the Boston Children's Hospital.