Researchers at the University of Southampton are conducting a study of near-death experiences.
People who experience near-death often see a bright light
Some people who have experienced near-death report seeing a tunnel or bright light, while others recall looking down from the ceiling at medical staff.
BBC News website readers have been sending in their stories.
A few years ago I was knocked of my cycle at a roundabout. The next 15 minutes are missing but I was aware that the light was blindingly pure and a richer blue than I have ever seen. I was surrounded by trees in reality, but the light was astounding and there was no noise whatsoever, there were no thoughts in my head, just wanting to be in the light. Then I realised that there was a person who I believed was my guardian angel. She was so tall that she could literally touch the sky, her body went on forever. She kept repeating "don't touch your head, don't touch your head". I did and suddenly pain and reality returned and I was aware of all that was going on. I was so sad because the light was gone and the guardian angel in reality was a lady who had stopped to help and she was not tall.
Jane, Bracknell, Berkshire, United Kingdom
I was involved in a head-to-head car accident a year ago, which caused me to lose consciousness and black out. I was taken to the hospital and was under cardiac arrest for 14 minutes. I remember vaguely seeing myself walking through a field with my deceased brother James. It felt like only a moment before I "awoke" in which I felt extreme pain shoot up through my entire body. I have now recovered but I still keep that memory with me.
Betsy Cromer, Boston, United States
I had an operation during which I was pumped with antibiotics that I was allergic to. I recall my soul slowly rising to the ceiling in the hospital and out of my body. My sisters were on either side of the bed begging the doctors to do something. As my body continued to rise a doctor gave me an injection. My soul (I believe) then began to lower itself back into my body. The following day I explained this to the doctor and he confirmed I had a near-death experience.
Gary Williams, London, United Kingdom
Several days before my wife died she had a cardiac arrest from which she was recovered. She told me that she was up against the ceiling watching the medical team working to save her. She found herself reflecting that she was leaving me and her daughter and made the tremendous conscious effort to come back to stay with us. But, she said, if it happened again she might not have the strength to return. Three days later it did and she didn't.
Gerard Mulholland, Paris, France
In the early 1980s I was overcome by carbon monoxide fumes from an electricity generator. I was rushed to Winchester hospital where my life was saved by a blood gas spectrum analyser. Meanwhile, I was going through a dark tunnel with beautiful lights and flowers at the end - with wonderful organ music! I didn't have a care in the world. Suddenly I stopped going through the tunnel and an American voice was shouting at me saying something like, "you're not going to die you bastard - it's not your time'! Apparently, when I woke up the young American doctor was surprised when I asked him, 'Do all Angels have American accents?'
Rob, Bournemouth, Dorset, United Kingdom
I had a heart attack, considerable pain and discomfort. My wife called 911. They arrived, loaded me into an ambulance and proceeded to the emergency room. My memory is that about a mile from home I went to sleep and began to dream. It was quite pleasant. There was a group of people talking and interacting pleasantly. I recognised no one but there was a purple tint to everything. I "woke up" just before we arrived at the emergency room. I told the EMTs that I had been napping. I was astonished that there were contact pads on me and my shirt had been cut off. They informed me I had flatlined. I can't describe it but what I experienced left me with a sense of peace and possibly less of a fear of dying.
Larry Huffman, Newton, NC, United States
I fell off a ledge backwards and hit my head and back on a rocky ground, when I was travelling in rural Laos a few years ago. I recall vividly seeing the back of my travelling companion's head, bending over me. And I remember feeling annoyed at him - because he didn't seem to be doing much to assist me. Then I opened my eyes, and saw him from the 'normal' angle.
Michele Legge, Canberra, Australia
Having worked as a nurse in a Critical Care Unit for 10 years, I have spoken with several patients directly after their near-death experiences. They all describe going down a brightly lit tunnel and experiencing complete peace and tranquillity. This is then followed by being pushed back down the tunnel again back to their bodies. One lady explained how she returned to her body, she then felt terrified for a few minutes because she could see the medical staff and got the impression they thought she was dead.
Shirley Learthart, Hastings, United Kingdom