Most love poems worry about what happens when the lovers are together. Sonnet 27 worries about what happens at bedtime. A man worn out by a day of labour and travel falls into bed expecting "dear repose" and finds that his body's shift has ended only so that his mind's can begin. The central idea is a double exhaustion. The beloved fills his thoughts so completely that he cannot sleep, yet the image that keeps him awake is also the only light he has in the dark.
The poem builds this from the vocabulary of work. "Toil," "travel," "journey" and "work" crowd the opening lines, and "travel" is a rich choice, since in Shakespeare's English it was close kin to "travail," meaning hard labour. Walking and drudgery are folded into one word. The fourth line makes the point neatly: "To work my mind, when body's work's expired." The repeated "work" shows that nothing has really stopped, and "expired" suggests the day's labour dying only for a new one to be born. Then the language turns religious. The thoughts make a "zealous pilgrimage" to the beloved, so the loved one becomes a shrine and the thinking a sacred duty. The speaker stays motionless in bed while something inside him sets out on the road. The phrase "from far where I abide" suggests, judging by the text, that the two are apart.
The second quatrain and the start of the third are full of paradoxes about seeing. His "drooping eyelids" are forced "open wide," and he looks at "darkness which the blind do see." A "sightless view" and the "soul's imaginary sight" follow, as if the poem is hunting for a way to describe seeing without eyes. At line nine, "Save that" turns the poem. Until then the night has been pure torment. Now the beloved's "shadow" arrives, a word that can mean a likeness or a ghostly figure, and perhaps hints that this is only a faint copy of the real person. The shadow lights the dark "like a jewel," making "black night beauteous, and her old face new." Night becomes an old woman given a miraculous makeover. "Ghastly," used just before, still carries some of its older ghostly terror, so the jewel shines against real fear.
The form quietly supports all of this. The sonnet follows the English pattern of three quatrains rhymed abab, cdcd, efef, then a couplet, in steady iambic pentameter. The rhythm is regular enough to feel like walking, one foot after another with no chance to stop. Yet the first line begins with a heavy stress on "Weary," so the tiredness lands before anything else. The final couplet returns to where the poem began: "by day my limbs, by night my mind." The two halves of that line balance like the two halves of a working day. The last line, "For thee, and for myself, no quiet find," puts the missing quiet at the very end, and "find" rhymes with "mind." The sound promises a discovery that the sense denies. The words "for thee, and for myself" also let love be both a gift and a need. He suffers for the beloved's sake and for his own.
What makes the sonnet worth reading is how truthfully it describes a familiar kind of sleeplessness. The cure and the sickness turn out to be the same thing. The vision that brightens the night is exactly what keeps him from resting in it. Shakespeare gives that loop a grand setting of pilgrimages, jewels and a rejuvenated night, but the experience underneath is ordinary. Anyone who has lain awake thinking about someone else will recognise it. The poem also makes love sound like work, something that does not clock off when the body does, and it manages this without ever complaining. The speaker seems almost proud of his sleepless labour.