Sleep is not much different with death, it's just sleep, you could say dead for a while. Dying is to sleep forever. God's Word s.w.t. :
وهو الذي يتوفاكم بالليل ويعلم ما جرحتم بالنهار ثم يبعثكم فيه ليقضى أجل مسمى ثم إليه مرجعكم ثم ينبئكم بما كنتم تعملون
Meaning:
"It is He who puts you to sleep at night and He knows what ye do in the daytime, then He awaketh you in the day to be perfected your prescribed age, then unto Allah ye return, and He shall tell you what which you used to do. " (Q.S Al-An'am: 60)