The Quran

اللَّهُ يَتَوَفَّى الأَنفُسَ حينَ مَوتِها وَالَّتي لَم تَمُت في مَنامِها ۖ فَيُمسِكُ الَّتي قَضىٰ عَلَيهَا المَوتَ وَيُرسِلُ الأُخرىٰ إِلىٰ أَجَلٍ مُسَمًّى ۚ إِنَّ في ذٰلِكَ لَآياتٍ لِقَومٍ يَتَفَكَّرونَ

Ali Unal

God takes the spirits at the time of the death of (the souls), and in their sleep those (of the ones) that have not died. He withholds (the spirits of) those for whom He has decreed death, and the rest He sends back (to their bodies to live on) for a term appointed by Him. Surely in that are signs (important lessons) for people who reflect and are mindful.¹

This verse draws our attention to a number of important facts, including the following:
It is not the human spirit but the human soul that dies. The soul (nafs) is, in one respect, the human being itself and, therefore, encompasses the spirit; and in another respect, it is the center or mechanism of worldly life. As for the spirit, it is the source of both bodily and spiritual life. It has an existence independent of the body, and it continues to live after death.
The human soul has various dimensions. These dimensions are mentioned by the scholars as the human spirit, the animal spirit, and the vegetable spirit. The human spirit also has dimensions which are described as the spirit that believes in and worships God (the spirits of the believers) and the spirit common to all people, which thinks, learns, loves, etc.
Death and sleep are identical in that during both, God takes the spirit. However, at death, God does not return the spirit to the body, thus causing the soul to die; while during sleep, He takes the spirit, but returns it if He has not ordained death for the soul. So, at death, the spirit leaves the body completely; but during sleep, it does not leave it completely and, therefore, continues its relation with the body. If the body still has some degree of animal life during sleep, it is because there is still a relation, however weak it is, between the body and the spirit during sleep.
Death is when the spirit leaves the body completely and begins another kind of life, which is called the intermediate life (of the grave) between this world and the Hereafter. The conditions of this life are determined by the person’s belief and deeds in the world. The spirit still has a relation with its body through the part of it that does not rot under the soil.