Mythological Twist

In the 1700s, it was a common practice to give an academic figure study a mythological twist. For example, this 1756 painting by Nicolas-Guy Brenet isn't just any reclining figure. It's "Sleeping Endymion." 

Endymion was a shepherd who had attracted the loving attention of the moon goddess Selene. She caused him to fall into an eternal sleep (with his eyes open) to preserve his beauty and youth. She would then be able to visit him every night.

Here's a similar pose but a different mythological setting. The painting, by Jean-Bernard Restout (1736-1796), is called either "Somnus," "Morpheus," or "Hypnos." Hypnos is the god of sleep or resides in a cave of eternal darkness. He is often shown with wings coming from his head, but here he looks more like an angel resting against his wings.

Adding these mythological layers can seem extraneous or gratuitous if the story doesn't guide the entire conception from the start. But when it's done thoughtfully, it offers both the artist and the viewer many new layers of feeling and association.

The problem for artists these days is that the audience is generally not familiar with the stories and the characters of the Greek and Roman mythology or of the Bible. An artist can count on everyone knowing what a cupid or a mermaid is, but viewers might not be as familiar with characters such as Sisyphus. Despite Hollywood's recent attempts to popularize mythic stories, the characters most people recognize tend to be comic book superheroes, which are trademarked and owned by big corporations.
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Here's another interpretation of Endymion
Wikipedia / Endymion
Wikipedia / Hypnos