(he takes) during their sleep. Those on whom He passed the decree of the death he keeps back (from returning to the body), but the rest He sends (to their bodies) to a term appointed. Verily in this are signs for those who reflect (al Zumar 39).
First of all let me analyze and define two important words mentioned in these Qur'anic verses namely al-Wafat and al-Mawt, respectively. Although the two words are used synonymously in our common speech in the Arabic language, so that both indicate death, yet they appear from these verses to be totally different. Al-Mawt indicates death and al-Wafat indicates what I may translate into 'retrieve the self or spirit' back to heaven. These two words being defined, the verse, then, clearly indicates that during sleep the self or the 'spirit' is retrieved by God just as with dying, as simple as that, without twisting words or introducing a trace of bias or prejudice with or against the Holy Book. They also indicate that if the body dies during sleep (i.e. something else happened to the body to cause death) the self or the spirit is not allowed to return back to the body. In other words, essentially the spirit does not reside in a dead body. A critical and gripping question arises: what happens to the body during sleep that cause death except for the leaving of the power that makes the difference between life and death that 'secret' endowed by Almighty God that makes matter turn it into a living object? For this secret power I reserved the nomination 'Soul'. And it does not require a lot of thinking to deduce then that a 'Human Being' is composed of three components that I call the "tripartite nature of man". These are the 'Body', the 'Soul' and the 'Spirit' (or the self) and we accordingly can deduce the following' Human Equations' :
Body + Soul + Spirit = a Living Human
Living Human -Spirit = a Living Body 'Sleeping Human , Human Being -(Spirit + Soul) = Dead Body
Human Being -Soul = Death and the automatic departure or exclusion of the Spirit as it, by its nature, cannot reside in a Dead Body. Distorted body (e.g. by accident) = death and the simultaneous departure of the Soul and Spirit as they both require an intact (viable) body.
It is worth noting that God used the word wafat in other verses to indicate Spirit retrieval during sleep:
It he who take your Spirit by night and has knowledge of an that you have done by day. By day does he raise you up again that a term appointed be fulfilled. In the end unto Him win your return, then He will show you the truth of an that you did. Al-Anam 60
Almighty God demanded that we search His signs both in the internal world of our body and the external world of the universe. He, in addition, through His revelation in the Holy Book to His beloved Prophet provided material for meditation and educated in-depth thought. In all times and every era since the revelation thinkers have and will always find material in the Holy Book for study. The Qur'an will continue to reveal its heavenly secrets till the Day of Judgment.