Brain | Clinical Death is a type of death occurs when the heart stops and can be revived with early medication. But, Biological Death is the type of death occurs only after 4–6 minutes of Clinical death. This is due to the brain getting an irreversible damage because of blood not flowing.
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When you die you know you are dead:

Study Shows Mind Still Works After Death

Biologists find working brains after the bodies show no signs of life

Science Section Staff
23 October, 2017

{And they ask you, [O Muhammad], about the soul. Say, “The soul is of the affair of my Lord. And mankind have not been given of knowledge except a little.”} [Surat Al-Israa 17:85].

Scientists say people are aware they’re dead because their consciousness continues to work after the body has stopped showing signs of life, Dave Maclean of the Independent reported last week.

“That means that, theoretically, someone may even hear medics announcing their own death,” claimed Dr. Sam Parnia, director of critical care and resuscitation research at NYU Langone School of Medicine in New York.

He and his team are looking at people who suffered cardiac arrest, technically died, but have later returned to life. It’s the largest study of its type ever.

Some patients say they had awareness of full conversations and seeing things that were going on around them, even after medics have pronounced them dead.

These accounts were then verified by the medical and nursing staff who were present at the time. Death is the point at which the heart no longer beats, and no blood flows to the brain.

Clinical vs Biological Death

Parnia said: “Technically, that’s how you get the time of death – it all depends on the moment when the heart stops. Once that happens, blood no longer circulates to the brain, which means brain function halts almost instantaneously.”

“You lose all your brain stem reflexes – your gag reflex, your pupil reflex, all that is gone,” he continued. However, there’s evidence to suggest that there’s a burst of brain energy as someone dies.

In 2013 researchers at the University of Michigan looked at the electrical signals inside the brains of nine anaesthetised rats having an induced heart attack. They saw activity patterns which are linked to a “hyper-alerted state” in the brief period after clinical death.

Parnia said: “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.”