Italian researchers have discovered an imbalance of serotonin in the brain stem may cause SIDS.

The brain stem coordinates the cardiovascular and respiratory systems and serotonin cells help control temperature regulation in the heart and other organs.

When researchers reduced serotonin functioning in mice, they were normal at first but then began to experience drops in heart rate and body temperature. More than half eventually died.

Researchers don't know if the results apply to humans, experts recommend placing babies on their back while they sleep to lower their risk.