When Does Death Occur?
Death is the permanent departure of the aethereal or spiritual soul from the material body.
The soul may depart temporarily from the body: during deep sleeping, fainting and losing consciousness. But in this case the material body is still alive: its systems keeps up working: the heart beats, the lungs respire, the alimentary system keeps digesting the food ..etc.
This is the important point:
Death does not occur unless: the viable fluid of the body is oxidized [by the angel of death] so that the soul will not be able anymore to contact with the material body; and man will die at his appointed death hour.
Or a major catastrophe happen to man: like cutting his head or a bullet hits his brain or suffocation or burning ..etc. And here also the soul will not be able to contact with the material body and wear it like you wear a complete tight suit. And here also man will die according to God's decree.
http://quran-ayat.com/man/index.htm#...ause_of_Death_