[This is my interpretation of the sonnet.]
The theme of Shakespeare's Sonnet 61 seems to be that of love's disruption to an otherwise normal life. In other words, the speaker notes that being in love is turning his life upside-down.
Shakespeare's sonnets generally organize his ideas in the first two quatrains (four-line stanzas). In the poem's first eight lines, the speaker asks the subject of the sonnet if it is "her" intention to mess up his life. Make note of the first quatrain:
Is it thy will, thy image should keep open
My heavy eyelids to the weary night?
Dost thou desire my slumbers should be broken,
While shadows like to thee do mock my sight?
The speaker is saying, "Is it your desire that images or thoughts of you keep me from falling asleep even when I'm exhausted? Do you want my sleep to be interrupted while I see shadows that look like you—though you're not here?"
The second stanza continues along the same lines:
Is it thy spirit that thou send'st from thee
So far from home into my deeds to pry,
To find out shames and idle hours in me,
The scope and tenure of thy jealousy?
The speaker asks, "Are you sending your spirit, far from you, to me in order to discover what I do with my time?" The "shame" may be that he is so enamored of her, and "idle hours" may speak to the fact that he lies about waiting till he can see her again. And does she send her spirit out of jealousy?
Notice how the poem's movement—the focus of the third quatrain (starting with line nine) changes. This is a hallmark of Shakespeare's structure of his sonnets. In this quatrain (stanza), the speaker notes that it is not her doing that has so disrupted his life—but his own doing.
O, no! thy love, though much, is not so great:
It is my love that keeps mine eye awake:
Mine own true love that doth my rest defeat,
To play the watchman ever for thy sake:
He grants that she does love him a great deal, but not so greatly that she would try to keep him awake at night. He says it is his own doing. His "own true love" keeps him from sleep, always watching out for her.
Shakespeare generally uses the last two lines, the rhyming couplet, to summarize the first twelve lines.
For thee watch I, whilst thou dost wake elsewhere,
From me far off, with others all too near.
The reader learns that the speaker watches for "her" while she is awake somewhere else, far from him, but much too close to others for his comfort!