What happens in the brain as a person dies?

What happens first, when the brain runs out of oxygen?

The brain is highly sensitive to any drop in blood flow and oxygen. A 2018 study in the Annals of Neurology, by Dreier and colleagues, recorded what happens when that flow ends. The team placed sensors in or on the brains of nine patients with devastating brain injuries. The patients had moved to comfort care only, and their breathing tubes were removed. The researchers then watched the brain’s oxygen fall.

In eight of the nine patients, normal electrical activity went quiet across the recorded area at the same time. This happened while oxygen was dropping steeply. The team called it “nonspreading depression,” because it did not travel from place to place. It simply stopped.

Then came a second event. A wave of electrical change, called a terminal spreading depolarization, began to move between the sensors. It started a median of 3.9 minutes after blood flow made its final drop. You can picture it as a slow tide rolling across the surface of the brain. That picture is an analogy. The real event is brain cells losing the electrical balance they need to work.

Why does this wave matter? In lab studies, the researchers explain, injury to nerve cells begins only when this kind of wave arrives. It typically starts 2 to 5 minutes after severe loss of blood flow. At first the change can be reversed if blood returns. If it does not, the damage becomes permanent. The researchers say knowing this timing could help doctors judge how long they have to restore blood flow.

Does the dying brain really “light up”?

You may have seen headlines saying the brain surges with activity at death. Here is what the studies found.

In 2013, Borjigin and colleagues at the University of Michigan recorded the brains of rats as their hearts were stopped in the lab. Within the first 30 seconds, they saw a brief surge of fast, tightly coordinated gamma waves. It came before the recording went flat. The fast activity even rose above the levels seen when the rats were awake.

In 2023, the same group looked at four patients who were in a coma and dying after their breathing support was withdrawn. Two of the four showed a sharp surge of gamma activity. It included stronger connections within a region at the back of the brain that some scientists think is critical for conscious processing. The surge was set off by the drop in oxygen and grew as the heart failed.

A 2022 report described one more case: an 87-year-old patient whose brain was being recorded when the heart stopped. After the heart stopped, overall activity fell. But fast gamma waves made up a larger share of what remained, and they stayed linked with slower rhythms even after blood flow to the brain had stopped.

What is the strongest objection?

The strongest objection is simple. These are very small studies. The human evidence comes from a handful of people, and only some of them showed a surge. The patients were in a coma or badly injured, so none could tell anyone what they felt. The 2022 authors note that injury and brain swelling may have shaped their recording.

So a burst of activity is not the same as an experience. Researchers call it a possible sign of heightened conscious processing, and they are careful with that wording. Some people link it to near-death experiences. None of these studies could test that link, because none of the patients survived to describe what they felt. And a brain recording cannot say whether anything continues after the brain stops. That question, which people of faith and skeptics answer in different ways, is taken up in Is there life after death?

Can a dying person still hear?

This may be the most practical question on this page. A 2020 study in Scientific Reports tested hospice patients while they could still respond, and again later, when they had become unresponsive. The team played patterns of tones and recorded brain responses. Most of the unresponsive patients still showed an automatic brain response to a change in tone. Some showed a response usually linked with noticing a change. The authors concluded that hearing may be one of the last senses to stop working.

The U.S. National Cancer Institute gives families similar advice. In the final days, it says, people may sleep more, answer slowly or not at all, and may seem confused. Most people can still hear after they can no longer speak, so it may help to keep talking to them and touching them. The same guide notes that a rattling sound in the breath can come in the last hours or days, and that it does not seem to be painful for the person.

What do people often get wrong?

“Brain death is a kind of coma.” It is not. In the United States, a 2023 guideline from several medical societies explains that the Uniform Determination of Death Act of 1981 lets doctors declare death in one of two ways. One is when the heart and lungs stop for good. The other is when all functions of the entire brain, including the brainstem, stop for good. Doctors check for three signs: coma, no brainstem reflexes, and no breathing during a special test. Coma is only one of the three. A person in a coma who still has brainstem reflexes, or who breathes on their own, is not brain dead.

“Every part of the brain dies the instant the heart stops.” Not quite. In a 2019 study in Nature, Yale researchers pumped a special fluid through pig brains that had been dead for up to four hours. Some blood vessel and cell functions came back. But the brains showed no overall electrical activity. So the death of brain cells is a process that unfolds over time, not a single instant, and science is still learning its timing.

“Sudden clear moments before death are well understood.” People sometimes report that someone with severe dementia suddenly seems clear and talks, especially near the end of life. A 2019 review called this “paradoxical lucidity.” It noted that the reports so far are mostly personal accounts and that the effect still needs to be confirmed with careful study.

How can you check this yourself?

• Count the patients. When a headline says the dying brain surges, look for how many people were studied. In the 2023 study it was four, and two showed the surge.
• Check the species. Some of the clearest data come from rats. Animal results can guide research, but they do not always match people.
• Look for the word “may.” The hearing study says hearing may be one of the last senses to go. That is careful language, and it is worth keeping.
• Read the care guides. The National Cancer Institute’s guide to the last days of life is free and written for families.

The answer stays the same after all this detail. As oxygen runs out, the brain goes quiet, a slow wave spreads through it, and damage sets in over minutes. Some brains show a short burst of fast activity along the way. What that moment is like from the inside, nobody knows yet. If you are thinking about your own death or someone else’s, Is it possible to make peace with the idea of dying? and Why does grief hurt so much, even physically? may help.