And thou wouldst have deemed them waking though they were asleep, and We caused them to turn over to the right and the left,
and their dog stretching out his paws on the threshold.
If thou hadst observed them closely thou hadst assuredly turned away from them in flight, and hadst been filled with awe of them. (18:18)

The only reasonable explanation for the turning to the right and to the left is because they were alive. Our movement during sleep is beyond our control, that is why Allah claims the act of the movement. Without turning it will damage the vein and whatever medical term are there. 

And they tarried in their Cave - three hundred years and add nine. (18:25)

Brittanica:
Phlebitis is inflamation of the wall of the vein.