﴿ ٱللَّهُ يَتَوَفَّى ٱلْأَنفُسَ حِينَ مَوْتِهَا وَٱلَّتِى لَمْ تَمُتْ فِى مَنَامِهَا ۖ فَيُمْسِكُ ٱلَّتِى قَضَىٰ عَلَيْهَا ٱلْمَوْتَ وَيُرْسِلُ ٱلْأُخْرَىٰٓ إِلَىٰٓ أَجَلٍۢ مُّسَمًّى ۚ إِنَّ فِى ذَٰلِكَ لَءَايَٰتٍۢ لِّقَوْمٍۢ يَتَفَكَّرُونَ ﴾

“It is God [alone that has this power - He] who causes all human beings to die at the time of their [bodily] death, and [causes to be as dead], during their sleep, those that have not yet died: thus, He withholds [from life] those upon whom He has decreed death, and lets the others go free for a term set [by Him]. In [all] this, behold, there are messages indeed for people who think!”

While sleeping a man loses consciousness. In this sense, sleep is similar to death. When he wakes up from sleep, he regains consciousness. This is a foretaste of resurrection after death. Through this system of nature, a man is shown today itself in an elementary manner how he will die and how he will rise up again. If he gives serious consideration to this matter, he will find in this very mundane event a great lesson of the Hereafter.