What's he saying?
"Weary with toil, I haste me to my bed, / The dear repose for limbs with travel tired;"
My body exhausted from traveling, I go to bed;
"But then begins a journey in my head / To work my mind, when body's work's expired:"
But even though I'm resting my body, my mind continues to work
"For then my thoughts - from far where I abide - / Intend a zealous pilgrimage to thee,"
Even though I am far away from you, my thoughts travel to where you are
"And keep my drooping eyelids open wide, / Looking on darkness which the blind do see:"
The thoughts of you keep my eyes wide open in the dark:
"Save that my soul's imaginary sight / Presents thy shadow to my sightless view,"
Unless I happen to imagine you there,
"Which, like a jewel hung in ghastly night, / Makes black night beauteous, and her old face new."
Which would bring beauty to the darkness around me.
"Lo! thus, by day my limbs, by night my mind, / For thee, and for myself, no quiet find."
During the day, my body yearns for you, and at night my mind is restless thinking about you.
Why is he saying it?
Sonnets 27-30 are meditative, focusing on the sleeplessness that comes with restless nights. This theme of a restless night spent thinking of a lover from whom the speaker is separated echoes traditional sonnets, for example Sidney's Sonnet 89 from Astrophel and Stella. Shakespeare is influenced by the themes of these sonnets, and might even be making fun of them.
The "zealous pilgrimage" upon which the speaker's thoughts embark in line 6 refers to a mental journey, as if his thoughts are capable of traveling physical distance like his body. Pilgrimages were taken to a holy place, like a church or a shrine, and often involved weeks of traveling by foot or on horseback to show devotion. In comparing thinking of the fair lord to a pilgrimage, the speaker implies that his devotion borders on religious faith.
The imagery of blindness permeates this sonnet, since the speaker is unable to use his eyes as he lies awake in the dark. As his eyelids are "drooping" with exhaustion, his thoughts keep his eyes wide open so that he can look "on darkness which the blind do see:" the night is so dense that it is as if he has no sense of sight at all. Instead, his imagination, or "my soul's imaginary sight," conjures images of his loved one in his mind.
In this sonnet, "shadow" is used to mean image. When the poet says that his imagination "presents thy shadow to my sightless view," he means it is as if the image of the fair lord is there in front of him, though in the darkness he physically sees nothing. Shakespeare plays with the meaning of this word, since "shadow" can also mean the darkness created by a person's presence. He also does so in Sonnet 43, lines 4-5: "Then thou, whose shadow shadows doth make bright, / How would thy shadow's form form happy show;" here, the literal and figurative definitions of "shadow" are juxtaposed.
The comparison of the "shadow" of the fair lord to a "jewel" in line 11 implies that "shadow" is meant to mean an image, rather than a darkness upon the dark of night. It is contrasted to the "ghastly night," which is personified as an old woman in line 12. The presence of the fair lord in the speaker's imagination has the effect of making night, which was so horrific, "beauteous," and the youth of the fair lord makes night's "old face new." These lines enforce the idea that youth and beauty are inextricably tied.