The turning point in Shakespeare's Sonnet 29 comes at the ninth line, which reads:
Yet in these thoughts myself almost despising,
The preceding eight lines of the sonnet describe the poet's painful worldly position, being short of money and probably in debt as well as being held in contempt by many who know him, along with his equally painful mental state, envying other more successful and talented men and almost despising himself because of the unfavorable comparison. Then he remembers the person to whom this poet is addressed and his mood begins to brighten, as he describes it this epiphany in one of the most beautiful images in all poetry.
Haply I think on thee, and then my state,
Like to the lark at break of day arising
From sullen earth, sings hymns at heaven's gate...
These inspired lines are effective for at least three reasons. One is that the change of mood is so sudden and unexpected. Another is that the words "break of day" seem to coincide with the the change in the poet's feelings when he happens to remember the person he loves. It is as if sunlight is breaking through the prevailing darkness of the first eight lines of the poem. And finally, the lines are effective because of the alliteration of "S" sounds in "sings hymns at heaven's gate," which suggest the sound of a lark singing. There are three "S" sounds in these five words, and the adjective "sullen" was probably chosen because it introduces an "S" sound at the beginning of the line.