Science - Sleeping is similar to dying

Three main areas of human brain are: Cerebrum, Cerebellum, and the Brainstem.  Medically speaking, as long as the Brainstem is not damaged and is still functioning, regardless of whether or not Cerebrum and Cerebellum are functioning, the patient can be kept alive.  Therefore, The brainstem plays a crucial role in sustaining life because it controls fundamental functions like breathing, heartbeat, and reflexes. Even if the cerebrum (responsible for higher cognitive functions like thought and consciousness) and the cerebellum (which coordinates movement and balance) are severely damaged or inactive, a person can be kept alive as long as the brainstem remains functional.

When you sleep, the Brainstem is almost inactive and some scientists argue that sleep is also a loss of consciousness due to the inactivity of the Brainstem neurons.  Additionally, when a person dies, the Brainstem becomes inactive.

Qur'an - Sleeping is similar to dying

ٱللَّهُ يَتَوَفَّى ٱلْأَنفُسَ حِينَ مَوْتِهَا وَٱلَّتِى لَمْ تَمُتْ فِى مَنَامِهَا فَيُمْسِكُ ٱلَّتِى قَضَىٰ عَلَيْهَا ٱلْمَوْتَ وَيُرْسِلُ ٱلْأُخْرَىٰٓ إِلَىٰٓ أَجَلٍ مُّسَمًّى

 إِنَّ فِى ذَٰلِكَ لَـَٔايَـٰتٍ لِّقَوْمٍ يَتَفَكَّرُونَ

It is Allah that takes the souls  (of men) at death; and those that die not (He takes) during their sleep:

those on whom He has passed the decree of death, He keeps back (from returning to life),

but the rest He sends (to their bodies) for a term appointed verily in this are Signs for those who reflect.  39:42

The Holy Qur'an defines a human being as a combination of 2 distinct entities:  body and soul.  The body belongs to this world.  After we die, the soul separates from this body and moves on to the next world.  Every time we sleep, Allah (SWT) takes away our souls.  Many hadith state that when the soul of a believer is taken away during the night, he/she may be awarded with some opportunities to learn depending on his/her degree of piety. The Holy Qur'an is stating that there is a resemblance between sleeping and dying regarding the soul.  Obviously this is not related to our bodies.