After Walter asks Ruth if something is wrong with her as a result of her chastising him shortly into the morning, Ruth responds with a simile:
No—I’m just sleepy as the devil.
The simile makes explicit how tired Ruth is, the same fatigue that permeates the room itself and that is often related to the experience of being a working-class Black American. In fact, referencing the devil in particular further emphasizes Ruth's fatigue: later in the play it will become clear that Mama is quite religious and that she keeps a Christian household to the best of her ability. Ruth appears to support Mama in this endeavor, even after she hits Beneatha. For Ruth to invoke the devil when explaining how tired she is, then, is particularly significant.
“Sleepy as the devil” also suggests that one’s behavior is sinful or improper on account of how tired they are. The reader can likely relate to the feeling that everything is harder when one is tired. In the case of the Youngers, however, they are constantly exhausted as a result of how challenging it is to live a disenfranchised life in a racist environment. They constantly work and face adversity. The simile links this fatigue to sinful actions; when one is sleepy as the devil, they are prone to act out of character. Ruth, for her part, keeps her composure throughout the play. Other characters, however, do act in a sinful manner, and this simile implicitly excuses those actions to some extent.