Orlando — To say retired Army Sgt. Carlos Cruz depends heavily on his service dog, Hannah, is an understatement. Cruz was diagnosed with post-traumatic stress disorder after returning from service in northern Afghanistan where he hunted for enemy explosive devices.

He acquired Hannah in January 2018 and says he's thankful for her every day. "It's amazing what she does for me," Cruz told CBS News. "I don't even know how she knows half the time. It's like an unspoken language, I guess you could say."

Scientists say there may be more to it than that.

About a 100 veterans and their service dogs are being studied by researchers at Purdue University of Indiana. As part of the unprecedented study, Cruz collected his saliva three times a day for three straight days this month to test his stress hormones.

Army Sgt. Carlos Cruz with Hannah CBS News

He also dons a wrist-band to track vital signs Hannah may actually be affecting and Hannah gets tracked, too. They'll be tested again in the summer.

"I think there are people out there who question whether or not service dogs actually help and they are looking for numbers and science," Dr. Maggie O'Haire told CBS News. O'Haire leads the Organization for Human-Animal Interaction Research and Education, a research group at Purdue that has undertaken this project.

O'Haire and her team are trying to find out if there is a chemical reaction service dogs ignite in their owners and vice versa — findings which could say for certain that dogs can help and why.

Cruz can't define it, but he knows Hannah helps.

"Sometimes just feeling her heartbeat and her breathing helps to calm me," he said.