Proserpine's Divine Calidore
Sleep. In the beautiful legend of Cupid and Psyche, by Apuleius, after Psyche had long wandered about searching for her lost Cupid, she is sent to Prosperine for “the casket of divine beauty,” which she was not to open till she came into the light of day. Psyche received the casket, but just as she was about to step on earth, she thought how much more Cupid would love her if she was divinely beautiful; so she opened the casket and found the calidore it contained was sleep, which instantly filled all her limbs with drowsiness, and she slept as it were the sleep of death.
This is the very perfection of allegory. Of course, sleep is the only beautifler of the weary and heart-sick; and this calidore Psyche found before Cupid again came to her.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894