It's hard to read the date on this Greiner sketch of a reclining male nude, but it was certainly during the 1880s, which would place it in the artist's student years.  Greiner was taught that the nude was foundational to art, and art historian Sepp Kern said of  him:

"The nude was central to his interests: like Klinger he saw it a the epitome of beauty in nature and believed that it should be the basis for all stylistic formation."