what it's about is he's been possesed by a demonic spirit that took up residence after he died in the hospital.
He doesnt know it's there yet, all he gets is the feeling and fleeting glances of shadows behind him as the demon thats taking over him gets a tighter and tighter hold over him.
The whole wispering thing is it's malevolence as it taunts him just on the edge of his hearing.

It has nothing to do with the night that part, it's a set up for latter.