The study authors from the Center for SIDS at Boston University Medical Center and Children's Hospital in Boston reviewed autopsy results of 41 SIDS babies and seven who died from known causes between 2004 and 2008. They found that SIDS babies had lower levels of serotonin and other related chemicals in their brainstems. They also had fewer receptors for the chemicals in their brain stems.
In practical terms, the researchers suggest that if a baby is put face down in the bed it begins to re-breathe carbon dioxide which is toxic. A normal baby could respond to that challenge, lift its head up, turn its head and arouse or wake up, but a baby who has a defect in those brain stem circuits that use serotonin, can't do that when challenged and they go on to die.
Researchers says the findings may give a biological basis for infants to be put to sleep on their backs.