What poem is Audrey Hepburn reciting in the movie Roman Holiday when she on sleep meds?

Audrey Hepburn is not reciting a poem in "Roman Holiday" while on sleep medication.

The scene you might be thinking of is when she is at the party at the American Embassy, where she is *pretending* to be drunk to escape the unwanted advances of a man. She recites a Shakespearean sonnet, specifically *Sonnet 116*, which begins with the line: "Let me not to the marriage of true minds."

This scene is humorous because the character is clearly not drunk, but rather simply using the sonnet as a way to politely and creatively escape the situation.