A small percent say they were able to hear full conversations, and even see what was happening around them.
"What tends to happen is that people who've had these very profound experiences may come back positively transformed - they become more altruistic, more engaged with helping others".
Researchers from New York's NYU Langone School of Medicine said people are aware that they died, and might be able to hear a doctor announcing their death.
New research at a highly regarded USA medical school suggests that some patients may retain awareness after their hearts have stopped and they are declared dead.
"They'll describe watching doctors and nurses working and they'll describe having awareness of full conversations, of visual things that were going on, that would otherwise not be known to them".
The team is examining what happens after the heart stops and how much brain the brain continues to function afterwards.
Dr. Parnia built his theory by studying American and European people who were first technically declared as dead, but then returned to life.
He said these recollections were then verified by medical and nursing staff who reported their patients, who were technically dead, could remember details of what they were saying.
The research explains that the medical definition of death is when the heart stops beating.
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As soon as the heart stops, blood is no longer circulating to the brain, which means the brain's function stops nearly instantaneously.
"You lose all your brain stem reflexes - your gag reflex, your pupil reflex, all that is gone".
A 2013 study from the University of MI studied brain signals of rats that were anesthetized and experienced a heart attack. The brain's cerebral cortex - which is responsible for thinking and processing information from the five senses - also instantly flatlines.
This triggers a chain reaction from a cellular process that will result in deaths of brain cells.
"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.
In all nine anesthetized rats, researchers saw activity patterns which are linked to a "hyper-alerted state" in the brief period after clinical death.
They're also looking for better methods of monitoring the brain beyond the threshold of death, Live Science reported, and improving resuscitation techniques to better prevent brain injury.
"In the same way that a group of researchers might be studying the qualitative nature of the human experience of "love", for instance, we're trying to understand the exact features that people experience when they go through death", Dr Parnia said.