Now scientists have discovered that a person's consciousness continues to work after the body has stopped showing signs of life - meaning they have awareness of their own death.
Dr Parnia and his team continue to investigate the pervasiveness of consciousness after death with twin studies in Europe and the United States who have suffered cardiac arrest, in the largest study of its kind.
New research at a highly regarded US medical school suggests that some patients may retain awareness after their hearts have stopped and they are declared dead.
"These patients would describe watching doctors and nurses working on them and they would portray awareness of full conversations and the visual things that were going on, which they should not have known", said the study's author Dr. Sam Parnia on Live Science, Thursday (10/19/2017).
The team is examining what happens after the heart stops and how much brain the brain continues to function afterwards.
So Parnia and his colleagues started the largest ever study of people in the U.S. and Europe who'd suffered a cardiac arrest - which is not the same as a heart attack - and were revived, and who reported having had "out of body" experiences during that time.
He mentioned how survivor's recollections were verified by medical and nursing staff, who had reported that some patients who were confirmed dead remembered details of what the medical team were saying while they were unconscious.
Death, in a medical sense, is when the heart stops beating and cuts off blood supply to the brain. "Once that happens, blood no longer circulates to the brain, which means brain function halts nearly instantaneously", Doctor Parnia said.
However, other scientists, who are not related to this work, share the idea that the brain still saves a little bit of energy after the body dies.
In the movie, a group of students take turns giving themselves a cardiac arrest to see what happens after their hearts stop.
"You lose all your brain stem reflexes - your gag reflex, your pupil reflex, all that is gone".
"The longer you're doing CPR, those brain cell death pathways are still happening-they're just happening at a slightly slower rate", he told Live Science. This means that within 2 to 20 seconds, no brainwaves will be detected on an electric monitor.
This eventually results in the death of the brain.
In all nine anesthetized rats, researchers saw activity patterns which are linked to a "hyper-alerted state" in the brief period after clinical death.
It is not the first time brain activity after death has been recorded.
"We're trying to understand the exact features that people experience when they go through death, because we understand that this is going to reflect the universal experience we're all going to have when we die", Parnia said.