What is the significance of the title in Long Day's Journey into Night by Eugene O'Neill?

O'Neill's play takes place during a twenty-four-hour period (or less) in which the problems of the Tyrone family are laid bare in stark detail. It is thus a "journey," as the title indicates, beginning in seeming placidity as the family start their day, and ending in virtual catastrophe the night of that same day .
Each of the characters is living in a self-created world of delusion. As the long day progresses, the family members are slowly forced to confront the reality of their failures. Mary is a drug addict; she has thought to hide this from her husband and sons but is ultimately unable to. Edmund has consumption (tuberculosis); he and the others have suspected this, but Mary, especially, has been in denial about it until the end of the play when Edmund announces it to her. Jamie, Edmund's elder brother, is an alcoholic and a failure in life. Tyrone is an alcoholic as well, reliving his past glory as an actor and refusing to accept the negative things about his wife, his sons, and himself. The entire family are in denial about all these facts; the title of the play refers to the journey, taking place from morning through to night, of their being forced out of the cocoon of denial to recognize the harsh reality of their suffering.
The title also symbolises the "journey" through which addicts go through on a daily basis. All of the Tyrones are using drugs or alcohol. For people with addiction problems, every day is a repetitive process in which they drink or use over and over, until night finally brings sleep. It's thus possible O'Neill intended the title as a metaphor for the disease of addiction.

In writing Long Day's Journey into Night, O'Neill deliberately adheres to the Aristotelian unities of place and time. The action takes place over the course of a single day in a single location, the Tyrone summer house in Connecticut. And a very long day it is, too. For over the course of the play, the various tensions between family members that have been simmering away beneath the surface for years finally come to the boil.
Mary Tyrone's babbling, drug-induced monologue, which ends the play, takes place around midnight. This is entirely appropriate in the overall scheme of things. For though midnight heralds the beginning of a new day, we know that, thanks to all the myriad problems encountered by each member of the Tyrone family, things will go on pretty much as they always have. There's no sense of resolution here; midnight may bring silence and stillness, but it's the silence and stillness of death and stupefaction. The long day's journey may have entered into night, but it hasn't ended there.