The song's imagery about an idyllic swimming trip are from John Bayley's memoir of his wife Iris Murdoch, Elegy for Iris. The passage recounts a happy time together before the late author's sad slide into dementia.
Iris Murdoch (1909-1999) was diagnosed with Alzheimer's disease in 1997 and died two years later in Oxford.
The sunrise ballad came from the writing sessions Alt-J did for British indie film Leave To Remain.
Bright Eyes' Conor Oberst, Lianne La Havas, Marika Hackman and Sivu were recruited to add vocals and whistling which were then spliced to fit between those sung by Joe Newman.