Eugene O'Neill wrote plays prolifically during his career.  Like most writers whose writing spans several years, his plays reflected changes in his life.  Thus his writing career can be said to represent three distinct periods.

O'Neill's first plays drew heavily from his life as a sailor.  Bound East for Cardiff, among others, reflected life on the sea.

O'Neill's middle plays were expressionistic rather than realist.   Drawing from the minds of Freud and Jung, O'Neill presented plays that represented the human condition through unconventional ways, more symbolic than realistic.  The Hairy Ape is one of the premiere examples of this period.

O'Neill's final plays were autobiographical, a type of catharsis, which reenactd his dysfunctional family life.  A Long Day's Journey Into Night reflects his own alcoholism combined with his mother's tragic addiction to morphine and his brother's fatal illness.