What is the significance of the simile O'Brien uses to explain his inability to move while standing in the field of mush? what is the simile?

The similie he uses is:

"Like waking from a summer nap, feeling lazy and sluggish"

It is significant because is it the exact opposite of how he really felt when he'd been there in the past........ it wasn't a summer nap and he hadn't felt lazy or sluggish. He'd felt guilty and weighed down by Kiowa's death.