Yes, exactly. It is more than sleep, though. Consider this passage from the story:

"He did not know he was so hard struck; that knowledge would come later, and never go. Grief is an artist of powers as various as the instruments upon which he plays his dirges for the dead, evoking from some the sharpest, shrillest notes, from others the low, grave chords that throb recurrent like the slow beating of a distant drum. Some natures it startles; some it stupefies."

He is essentially in shock, not asleep, and it is made worse by the fact that he doesn't know it. He can't wake up; he is in full retreat from life, hit by something he has no experience with. His mind then turns the noise of the panther into something from his dreams.