The cyclical movement of time, as represented by the progress of day to night, is one of the central symbols of Long Day's Journey. The Tyrone family is caught in a similar cycle. They attack each other, they feel bad, they apologize, they say something mean, they feel bad, they apologize…it's the family feud that never ends. The whole play is built around these cycles.
Think also about the nature of addiction, and of Mary's morphine addiction in particular. Abuse, regret, back on the wagon, fall off the wagon, abuse, regret…The play might draw to a close, but we have a feeling the cycles are never-ending. We talk about all this in greater detail in "What's Up with the Title?" Check it out.