I was in the Blessed Sacrament chapel at St Stephen's a couple of days ago, listening distractedly to the sermon echoing in the body of the cathedral behind me, as I drifted off sleepily. The almost indecipherable words commanding attention from some unseen source reminded me of another sensation. I suddenly realised that one of the defining characteristics of the state of drifting into sleep is a similar loss of precision in things heard.
As you slip over the edge, some sounds retain a grip on the attention longer than the rest; familiar voices, for example. It's not that these foreground sounds echo—indeed they seem to become sharper—but that the auditory background becomes blurred, so that the voice that arrests your melting attention becomes detached, and drifts off above the blurring and echoing sounds below. It was a sensation frequently noticed and immediately forgotten, brought back by this auditory accident before the tabernacle.