‘Lullaby’ is a poem by the Anglo-American poet W. H. Auden (1907-73). It was published in 1937, when Auden was still living in England (he would depart for the United States in early 1939). The poem is an example of a love poem, but there are a number of things worth noting about it, which we’ll come to in the analysis of ‘Lullaby’ below. You can read the poem here.
‘Lullaby’: let’s start with the title. A lullaby is, of course, a song sung to soothe someone to sleep, especially a baby or young child. Immediately, in that famous opening line (‘Lay your sleeping head, my love …’), Auden challenges our expectations of the lullaby: the person he addresses is a lover rather than a child, and he is addressing them while they are already asleep. We know from Auden’s biography that he was gay at a time when homosexuality was still criminalised in the UK (and would be until the reforms of 1967); so we can deduce that the addressee of ‘Lullaby’, that ‘love’ to whom Auden addresses his words, is another man.
Interestingly, over at the British Library website, Roz Kaveney notes that at the time Auden wrote ‘Lullaby’, he was trying to seduce the composer Benjamin Britten (who was also gay). Could this poem, written by one of the twentieth century’s greatest English poets, have 
been inspired by one of that century’s greatest composers? It seems possible. Auden and Britten would go on to collaborate on a number of projects together, although it seems probable that Auden never managed to effect a partnership with Britten on a romantic plane.
Whether or not Britten was the intended recipient, ‘Lullaby’ is a poem which sees Auden subverting a number of conventions of the love poem. In that second line, his description of himself as ‘faithless’ suggests someone who has lost faith in ‘love’ as an idea but is nevertheless committed to living in this moment with his beloved. ‘Faithless’ also, of course, summons its opposite, ‘faithful’: a hunch which is confirmed when we are told that both certainty and ‘fidelity’ disappear at ‘the stroke of midnight’. Is this a casual fling between two lovers, albeit one whose passions are intensely felt during this particular moment?
Let’s go through the poem stanza by stanza and summarise it in a little more detail. The first stanza sees Auden (or the lyric speaker of the poem, who may not be Auden; but we’ll call him Auden for the sake of brevity and clarity) addressing his (male) beloved as his lover sleeps with his head on Auden’s arm. Auden knows that beauty is fleeting: time and illness destroy the beauty of youth, and death soon arrives to prove that the young are not young (‘the child’) for long. Our life span is soon over. But what matters is the here and now. Auden now addresses someone else – almost some god or higher power – asking that this ‘living creature’ be allowed to remain in his arms until morning. Auden is aware that the ‘creature’ he loves is but a mere mortal, and is not perfect, ‘guilty’ of man sins and moral flaws; but to him (and beauty is, after all, in the eye of the beholder) he is completely beautiful.
In the second stanza, Auden turns to more supernatural and divine ideas of love. The syntax and punctuation are more complex here, but Auden is saying that lovers who lie upon the ‘slope’ or hill of Venus, the Roman goddess of love, are sent a ‘grave’ vision by that goddess, a vision of ‘universal’ love. This is ‘grave’ because it is deeply felt, and deeply serious: love is one of the most important things in our lives. Even the ‘hermit’, living alone and cut off from society, is affected by love: the ‘glaciers’ and ‘rocks’ of his stone-cold heart can also be thawed and warmed by love’s power.
In the third stanza, Auden continues to elevate this moment or ‘this night’ as significant and filled with meaning for him. Certainty (e.g. that one’s lover will remain faithful to one) and fidelity (i.e. one’s own faithfulness to one’s lover) are both fleeting, and pass like the tolling of a bell. Everything that has to be paid – and all pleasures, as the Earl of Rochester knew, carry a debt – will be paid: Auden knows that. But what matters is tonight, and he, in a sentiment that pre-empts Aerosmith, doesn’t want to miss a thing. (The reference to the ‘pedantic boring cry’ of ‘fashionable madmen’ is cryptic, but is probably, given the poem’s 1930s context, an allusion to Hitler, Mussolini, Franco, and other political leaders: in ‘September 1, 1939’, Auden refers to Hitler as a ‘psychopathic god’. There’s also possibly a recollection of John Donne’s great poem ‘The Sun Rising’, which begins with Donne lying in bed with his lover at ‘break of day’, and reprimanding the sun as a ‘saucy pedantic wretch’ for shining through the curtains and telling him and his lover they have to get up.)
In the final stanza of ‘Lullaby’, Auden continues the sentiment seen in the previous stanza: all things must pass. He picks up on three things already specifically mentioned: the ‘beauty’ of his lover, the ‘stroke of midnight’, and the ‘vision’ that Venus sends to lovers lying in post-coital bliss on her ancient hill. If morning must come – as he knows it must – Auden asks that it at least be the dawn of a day which offers hope and blessing to the lovers. The poem is hopeful, but throughout Auden remains aware of the fragility and impermanence of all human relationships.