First of all, Long Day's Journey Into Night is literally that: a very long day that eventually fades into night. It's been happening since the Earth started to rotate, and it won't quit until the spinning finally stops. Umm, so doesn't day turn into night over the course of every 24 hour period? What special significance could such a title have? We have a few theories.
Let's start with the Symbolism 101 approach. You've got day, and you've got night. Day = sunshine = hope. Night = darkness = despair. (More about this in "Symbol, Images, Allegory.") The Tyrones start off the play in the bright morning sun, hoping against hope that Mary has finally kicked her addiction to morphine. By the end, it's the dead of night, and poor Mary is back on the drug. The promise of a happy healthy mom has been crushed. This has been going on for a while. Mary has been trying to kick the stuff for years. Every time, though, she loses control. It's a vicious cycle.
A cycle, huh? Key word alert! It seems like we were just talking about another kind of cycle...What was it?…Ah!...Day and night. So, could it be that the title is referencing Mary's repeated descents into addiction? Could it be that the title gives us a major clue into understanding the play? There's a good chance of it.
The family is caught in a series of cycles. They launch the same attacks over and over again – Tyrone is a cheap skate, Mary is a morphine addict, Jamie is an alcoholic. Edmund's birth and tuberculosis are even used against him at times. These attacks are only part of the cycle though. After each one, the Tyrones forgive each other. We see hate then love, and love then hate – dark then light, light then dark. Get it?
Just as the Moon circles the Earth, and the Earth orbits the Sun, and the Sun revolves around whatever it is that it revolves around, so to go the Tyrones. They are trapped together repeating the same dark sins while reaching for the same bright absolutions. The title seems to reveal the play as cycles within cycles within cycles within…