The master mentioned in the final line of the Robert Bly's poem "Waking from Sleep" is actually sleep itself.  Bly personifies sleep by making it "our master" as if sleep controls and demands rest from its human counterparts.  The majority of the poem revels in the burst of action of the human spirit and body upon waking from a long rest.  Bly uses figurative language, like simile, hyperbole, metaphor, and imagery to convey the depth of the rest, as in "the country has slept the whole winter," and the subsequent energy upon release, "our whole body is like a harbor at dawn." 

Robert Bly first published this poem in 1962, at a time when the U.S. was still committed in war with Vietnam, so the poem could be interpreted as Bly's vision for how the U.S. will feel upon ending the war.  This poem is very much about rejuvenation of body and spirit; Bly's reference to our master, "sleep" which has dulled the senses and rendered the U.S. lifeless and numb could be interpreted as an extended metaphor for the United States' involvement in Vietnam.