A note of ambiguity or unease often gives a piquant quality to German adaptations of the Renaissance ideal. Cranach's painting of a classical nymph represents an Italian theme but gives it a moralizing twist common to late Gothic courtly and amorous subjects.
The nymph reclines beside a spring, perhaps a reference to a legendary ancient Roman fountain with which a Latin verse was associated. The text was translated by Alexander Pope in 1725:
Nymph of the grot, these springs I keep, And to the murmurs of these waters sleep; Ah, spare my slumbers, gently tread the cave! And drink in silence, or in silence lave!
The inscription on this painting -- I am the nymph of the sacred spring, do not disturb my sleep. I am resting -- may be an allusion to the poem. Though exposed by modern scholarship as a fifteenth-century counterfeit, the poem influenced Italian garden decoration, which not infrequently included fountains with attendant reclining nymphs. However, the proportions of Cranach's nude are more Gothic than classical, and the robe on which she rests her head is that of a German court lady. Far from sleeping, she admires herself beguilingly through lowered eyelids. The painting is intended both as an enticement and a warning to Cranach's sophisticated patrons.
More information on this painting can be found in the Gallery publication German Paintings of the Fifteenth through Seventeenth Centuries, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/german-painting-fifteenth-through-seventeenth-centuries.pdf