Anyone who has lain awake desperate for sleep will recognize the situation here. The speaker is stuck in insomnia, and the poem treats it as a kind of torment: clawing at the air, gasping for dreams, drifting through imagined versions of other lives, the "might-have-beens" that keep surfacing in the dark.
The repeated image of "stepping in that stream / And stepping in that stream again, again" captures the loop of the speaker's thoughts. The mind keeps returning to the same places, whether the speaker wants it to or not. The night feels "thick" with living and reliving, and the speaker is weighed down, "prone" and sick for rest.
The mood is exhausted, restless, and a little claustrophobic. The final line is striking. The speaker is "breathing, 'Wake,'" which suggests that even waking would be a relief, a way out of a night that is neither sleep nor real rest.