One of the interpretive themes of LOST was heaven and hell. Some fans felt that the main characters were in hell, symbolized the by the island and its various monsters, sinners and punishments. Others thought that the characters found their heaven in the sideways world, which ran concurrently in the Season 6 series - - - but if you look at Alpert's back story, it had been running for eternity.
The concept of duality - - - two different planes of existence - - - is also a reoccurring theme. Many scientists believe in the time-space continuum contains multiverses, where each action creates a new and slightly different universe based upon those probable effects of an action.
But there is a simple way of looking at the LOST's split story personality.
From a spiritual perspective, each living person has two states: an awaken state and a sleeping state. When you are awake, you are living and experiencing the good and bad of real life. Over time, daily routines like work and family become grinds. The repetitive nature of living is a drain upon one's creative, adventurous curiosities. In order to fulfill that need, people tend to have deeper, more complex dreams when they sleep. Sleep is a person's way of repairing and re-energizing their body. But it can also be a needed fantasy diversion to a person's "perfect world."
In fact, people sometimes can get "lost" in their dreams. They may not be able to tell what is real and what is fantasy. The illusions in dreams become delusions in real life. And this creates personality disorders and mental mistakes, deep problems at work or with family. A prime example could be Locke, who drifted fantasy games into his real life to the point where he gave up his normal existence to join a commune in the hope of finding a "family"
It can be said that when people are awake in their boring, rotten, habitual work world, they are living in hell. For most of us, we cannot change the relentlessness of this existence. We are trapped by circumstance and obligation.
But when we totally get away from the daily routine, our dreams can appear to be heavenly. Our fantasies make us the star of our own movie. We can be the hero, the lover, the protector, the superhero, the leader, the greatest person in the world.
LOST's island world could be considered the collision of both the heaven and hell aspects of a person's existence. It contains fragmented bits of both worlds which cannot exist together. The show rarely showed anyone sleeping - - - insomnia was rampant. It could be a clue that the characters were trapped in a limbo between being awake and being asleep - - - a place ruled by their nightmares.