The phrase "tender is the night" comes from Keats's poem "Ode to a Nightingale." Several lines of the ode also form the epigraph to the novel, and many of its lines clearly apply to the atmosphere and characters, particularly the images of intoxication and mental breakdown.

The essential meaning of the phrase is clear enough: the night is tender, which is to say that it is both vulnerable and nurturing. The darkness of night hides a multitude of sins and wounds, and the characters in Fitzgerald's novel are fragile people, sorely in need of such concealment. This is particularly true of Nicole, who has been damaged by her father's abuse and suffers from neurosis.

Dick is also a vulnerable character and becomes more vulnerable over the course of the narrative. At first, he believes that he can care for Nicole and offer her some stability, but, in fact, he cannot even look after himself and becomes dependent on alcohol.

Superficially, these are glamorous, intriguing people, whose lives are enviable. It is through Fitzgerald's acute psychological examination that the reader comes to see how flawed and fragile they are. One might say that the author shines a merciless light upon his characters and that they cannot endure such exposure, since these are people who need the tenderness of night to preserve their illusions.

F. Scott Fitzgerald took the title of his 1934 novel Tender Is the Night from John Keats’s 1819 poem “Ode to a Nightingale.” The novel’s title incorporates the idea of the night as being a fundamentally different environment than the day. This applies both to covering things that are visible by day and allowing delicate things to flourish that would be harmed by the bright sun. In Keats’s poem, the beautiful song of the nightingale is made all the more precious because the bird that produces it sings only at night, and those who hear it cannot see it. Keats uses the nocturnal bird’s songs as a metaphor for creative production—in his case, poetry. In the Keats lines, the speaker addresses the bird directly:

Already with thee! tender is the night,

And haply the Queen-Moon is on her throne,

Cluster'd around by all her starry Fays;

But here there is no light,

Save what from heaven is with the breezes blown

Through verdurous glooms and winding mossy ways.

In Fitzgerald’s novel, the metaphorical night may be tender as it helps the characters hide their many problems and keep their secrets. In particular, Nicole has a deep need to hide the childhood sexual abuse she suffered from her father. Dick aims to repress the conflicts he experiences between his roles as her doctor and her husband, as well as his guilt over depending on her family money. His darkness also includes being overtaken by alcoholism.

In addition, the location where “there is no light” and the “glooms” that hang over all the characters can refer to the general state of postwar Europe that is the novel’s setting. The Riviera locations provide a contrast between the characters’ daytime seaside activities and the nocturnal escapades.