The meaning and implications of Plath's poetry is usually profound.  "Night Shift" is no different.  There are many layers to this poem, starting from the sound in the first stanza.  This sound ends up constructing the entire narrative of the poem.  The speaker of the poem, perhaps someone like Plath herself, hears it.  The first stanza describes this sound as something that resonates:  "that clangor/Far off, not blood in the ears/Drumming up and fever."  It is a sound that impacts the evening air, something heard and cannot be blocked out.  Plath injects the "Native" to the "suburbs" of the poem's setting.  This sound is something intrinsic to the people living there.  The sound has become part of the environment:  "These stilled suburbs nobody/ Startled at it, though the sound/ Shook the ground with its pounding."  

The speaker of the poem investigates the source of the sound.  It is at this point where it becomes clear from where the sound is coming: 

Framed in windows of Main Street’s
Silver factory, immense
Hammers hoisted, wheels turning,
Stalled, let fall their vertical
Tonnage of metal and wood;
Stunned in marrow.

The presence of the factory and the lifeblood of the factory, the workers, is where the sound originates. The machinery that produces something undefined makes the sound, defining everyone who comes into contact with it.  The sound is ongoing, "indefatigable" and reflective of a condition that transcends being in the world. The sounds seems to define it.  The "Night Shift" aspect of the poem reflects how this sound and its source are embedded, ongoing and relentless in what it done and what is represented.  The individual who notices is almost dwindles in comparison to its magnitude and presence, one that underscores consciousness in the town and those in it.