The lyrics are by DuBose Heyward, the author of the novel Porgy on which the opera was based, and Ira Gershwin.
[3] Gershwin began composing the song in December 1933, attempting to create his own spiritual in the style of the African American folk music of the period.
That rendition finished at #52 in AFI's 100 Years...100 Songs survey of top tunes in American cinema.
[7][8] Heyward's inspiration for the lyrics was the southern folk spiritual-lullaby "All My Trials", of which he had Clara sing a snippet in his play Porgy.
This is reinforced by his extensive use (one exception: the note B under the word "high") of the pentatonic scale (C–D–E–G–A) in the context of the A minor tonality and a slow-moving harmonic progression that suggests a "blues".