What Happens After You Die? The Brain Keeps Working Long Enough for Thoughts to Form

A scene from the movie "Flatliners," in which a group of medical students purposely stop each others hearts. Michael Gibson/Sony Pictures

Few things seem more absolute than death. But when it comes to actually pinpointing the moment when a person goes from alive to dead, drawing the line is a little more fraught.

The current medical definition of death is when the heart stops beating. In an interview with LiveScience, NYU Langone's Sam Parnia, a pulmonologist who studies resuscitation, notes that from that point, it takes up to 20 seconds before brain waves are no longer detectable. According to Parnia, this sets off a longer chain of processes that leads to brain death. "If you manage to restart the heart, which is what CPR attempts to do, you'll gradually start to get the brain functioning again. The longer you're doing CPR, those brain cell death pathways are still happening—they're just happening at a slightly slower rate," Parnia told LiveScience.

In cases of cardiac arrest, doctors usually are not able to restore blood flow to the brain. That's why the experiences of people who are successfully resuscitated are so fascinating.

The LiveScience story taps into a vast history of efforts to define death and to understand what are commonly called "near-death experiences." In a 2014 study, physicians interviewed over 100 people who had gone into cardiac arrest and been successfully resuscitated: 39 percent of them described a perception of awareness, even if they couldn't recount specifics. One woman in a similar, earlier study, described a feeling of floating above her body and watching as the doctor called a "code blue."

The human brain. Matt Cardy/Getty Images

These cases blur the line and challenge a definition that might seem clearcut to most people, but continue to be the subject of ongoing debate. The cardiologist Haider Warraich lays out the debate in bioethics over whether decapitation, separating a head from its body, automatically equals death. Warraich also mentions the case of doctors in the 1800s trying to distinguish life from death by plunging needles attached to small flags into cadavers' chests (if the flag fluttered the heart was beating and the person was considered alive).

To try and prove their point that a creature could be "alive" without its head, a group of researchers in the 1990s beheaded a pregnant sheep and connected her body to a breathing machine. They delivered the sheep's baby by C-section. "There is no ambiguity here: the sheep remained alive during the experiment." the researchers wrote, concluding "decapitated animals are not necessarily dead."

And these definitions aren't chosen in a vacuum. Warraich writes that the decision to draw the line at "brain death" was motivated by the advent of organ transplants. Once organ transplants became possible, doctors needed a clear-cut marker of when a person was "dead enough" to remove their organs. The doctor who performed the first heart transplant in South Africa gave the donor a lethal injection to remove any room for debate.