Long Day's Journey into Night is a modernist play. Modernism focuses on experimenting with literary forms to highlight twentieth century fragmentation and alienation. Further, in a society that seemed shattered after World War I, modernism called into question the "givens" of the past.
In Long Day's Journey, the Tyrone family is fragmented. While Mary is most obviously caught in her own world of morphine addiction, all the family members are at odds with each other. Tyrone and Jamie are bitterly estranged, as are Jamie and Edmund, and Tyrone and Edmund. Each looks at life from his own self-justifying perspective, and each is alienated from the other, reflecting the modern condition of aloneness. In modernist form, too, the play does not offer the comfort of an overarching meaning, but leaves the audience to assemble the pieces into some form of subjective truth.
The play questions old "givens," such as upending the ideal of the happy family dear to the Victorian and Edwardian heart, showing this family not as a bastion of safety and security but as a deeply destructive force, lacerating its members. The self-absorbed, alienated, and ironically-named Mary—as she appears in ghostly white, lost in morphine hazes—becomes a parody of the Victorian angel of the home, an inverse of the self-sacrificial Victorian mother figure. Likewise Edmund, but also Jamie, upset Tyrone's Roman Catholic beliefs with their atheism. The younger generation, like good modernists, reject the faith of the past as a false comfort in a world that has come to seem increasingly meaningless.
The theme of meaninglessness also shows the play's use of naturalism, the post-Darwinian idea that the universe is random and that humans are nothing more than products of their environment, rather than creatures formed of God who can transcend their circumstances. Mary expresses this view when she says to Edmund of Jamie:
He can’t help being what the past has made him. Any more than your father can. Or you. Or I.