Near-death experiences like seeing a bright light at the end of a tunnel, feelings of peace and joy may occur because of high carbon dioxide levels in the blood, according to researchers writing BioMed Central's open access journal Critical Care.

Up to a quarter of cardiac arrest patients have reported mysterious experiences, which might include an out of body feeling or the sensation of looking down from the ceiling to the room below.

In the past, it was theorized that the phenomenon could be due to drugs or religious beliefs. But when Slovenian researchers at the University of Maribor examined 52 cardiac arrest patients, they discovered that levels of the colorless chemical compound were significantly higher in the 11 patients who said they had had a near-death experience.

Some of the cardiac arrest patients who report a near-death experience - the figure ranges from one in 10 to nearly 25% - have said they felt overwhelmingly peaceful and tranquil, while others said they could see a mystical entity or bright light.

It's known that inhaling carbon dioxide can bring on a hallucinatory experience not dissimilar to the type of feeling reported by the patients. Still a mystery is whether the higher levels of carbon dioxide among the group of 11 patients were due to the cardiac arrest or pre-existing.

It had been theorized that anoxia, a condition in which oxygen-deprived cells die - might be the reason why near-death experiences happen, but this was not of statistical significance among the patients in this study.