Years ago, I noted that the "Day and night / Night and day" pairs are temporal merisms; while these two opposites are named, the sense is actually "all the time." This effect is compounded because the order is reversed between the two pairs (although the second isn't really a pair; the whole line is "Night and day and night"). This morning, I realized that this effect is also emphasized musically because the "day" and "night" of each pair occur alternately on and off the beat. In the line "Day and night," "day" is sung on an upbeat, and "night" is sung on a downbeat. In the following "Night and day," "night" is sung on the upbeat, and "day" is sung on the downbeat. Both musically and rhetorically, then, these opposites and inversions indicate an entirety.
