Dorothy had made a long trek down the infamous yellow brick road. She had met a talking tin man, animated scarecrow and a giant lion who walked on two legs. Skipping and singing down the road, she had battles with the wicked witch, all along wishing she were back home in Kansas. Just as she was about to meet the Wizard, who would surely grant her a one-way ticket back home, she grew tired while walking through a field of poppies.
The poppies in this field must have been extremely potent, as Dorothy's quest to finally meet the wizard was surely more pressing than a nap. The field seemingly stretched on forever, and the poppies looked lush, fragrant and colorful. However temporary her nap was, it took her to the next level of her adventure.
Poppy seeds are considered an opiate, which helps explain why Dorothy became so relaxed that she could nap in the middle of the day. The psychological effects of opium have been explored and exploited for literally thousands of years. Opium preparations were often used as a painkiller for wounded soldiers of the Civil War. It's easy to see how the creator of this film found adding a beautiful field of opiate poppies to an already dream-induced adventure the perfect cinematic choice.
Her sleep was so peaceful she surely must have dreamed, even though the end of the film reveals that the entire adventure was a dream. The field of poppies was a visual representation of a dream within a dream and, as Freud would have said, very symbolic. Although partaking in excessive opiate pleasure can be extremely dangerous, Dorothy needed to fantasize in that field of poppies so that she could believe in the power of her imagination, which was necessary for getting back home. She was home all along, of course, but not yet living her life to the fullest. The nap in the poppy field, however potentially dangerous, enabled her to refocus and let go, and ultimately helped enrich her life.