The permit from the National Park Service to make it possible:
Ilyse Iris Magy...is hereby authorized...to use the following described land or facilities in the above named area:
Gateway Arch Grounds (grassy area located in between the legs of the Gateway Arch)
For the purpose(s) of: Gathering of approximately 50 participants...for an organized collective "nap" under the Gateway Arch. The Gateway Arch was chosen as the perfect site for your activity because it symbolizes the American dream, which ties into the purpose of your "nap," which is to serve as a "metaphor for the American Dream, engaging notions of humanity, collectivity, and brotherhood." Because your activity ties into the purpose of this park and does not constitute a disruption to normal visitor services/activities, your activity is authorized under this permit. Your group may sit or rest peacefully on a blanket that is approximately 7x60 feet long. No other equipment or setup is authorized. The blanket must be rolled up immediately upon conclusion of your activity.
Visit this page for more media.