This poem consists of five stanzas of six lines each. The rhyme scheme for each stanza is aabccb. The lines vary in length from three to nine syllables, and the rhythms vary from anapestic (line 1) to iambic (lines 19 and 20) to the rhythms of normal speech. Despite the consistent rhyme scheme, the frequent use of enjambment (lines that carry over to the next line without a pause or stop) makes the rhyming less noticeable. Thus, with the varied line lengths and rhythms and the rhymes that aren't emphasized, the poem feels more like a contemplative meditation than verse.
One of the most obvious poetic devices in the poem is metaphor. The poem begins by likening sleep to a deep forest. The path that leads to the forest ends in a blur, and travelers sink in, as if it were a swamp or pit. The final stanza presents metaphors within a symbol. The forest, which is a symbol for sleep, is compared to clouds—"cloudy foliage lowers"—and a piece of furniture—"shelf above shelf." This reminds one of the levels of sleep that one falls into, going deeper and deeper into unconsciousness.
Other comparisons that are not exactly metaphors occur in stanza four, where the poet compares the desire for sleep to the desire for an interesting book or a beloved person, and in stanza three, where the satisfaction gained from sleep is compared to the satisfaction of performing a noble deed.
Other devices the poet uses are near rhyme and paradox. Although most rhymes used in the poem are strong rhymes, in the third stanza the poet makes use of identical rhyme (ends/ends) and near rhyme (trouble/noble; bitter/sweeter). A paradox occurs in the final stanza where the poet speaks of hearing sleep's silence.
The poem's varied rhythms and line lengths, its near rhymes juxtaposed with identical rhyme, and the paradox in the final stanza all work together to convey the speaker's mixed feelings toward sleep.
Thomas uses enjambment, the breaking of a phrase into multiple lines or verses, to evoke a certain mood in the reader--namely, one which matches the tone he is using. As an example, in the first verse, he uses enjambment to help the reader feel as lost or disjointed as he does at the time just before he falls asleep:
I have come to the borders of sleep,
The unfathomable deep
Forest where all must lose
Their way, however straight,
Or winding, soon or late;
They cannot choose.
Themes in this poem include sleep: first defined as resting, alluring as a deep, dark forest, and then as death, unavoidable yet perhaps pleasant after the difficult tasks of life. Thomas refers to the towering foliage in the final verse, using imagery as he states that it is structured "shelf above shelf." He then states "That I may lose my way/And myself," which indicates a surrendering to either death or sleep, whichever theme by this point that the reader feels is most congruent. It seems that when the author suggests the "foliage lowers," he might be suggesting that nature is enveloping him once more, as we return to dust in death.
As with much of Thomas's poetry, this piece deals vaguely in human alienation, or at least solitude, as man must go to sleep--or his death--essentially alone.