Was Yeats thinking of sleeping "dreams" or future aspirations, though? I also wonder if the brain's "self-cleaning" process (which is what dreams are^) would function any longer if you had conscious control over it. Maybe that would actually turn out to have harmful side effects to your health? I've thought of this before when my brother-in-law talks about lucid dreaming.

^ https://www.nih.gov/news-events/nih-research-matters/how-sle...