This is still a popular theory/explanation for LOST: Dreams.
The whole show was an elaborate dream in the mind of one character.
The most likely candidate to be the series Dreamer would be Hurley. He had mental issues. He was a loner. He liked fantasy elements such as comic books and Star Wars. During the early seasons, ABC requested script changes because the story line was getting too clear that it was all in Hurley's head. It was around the time of the "Dave" episode, where Hurley had a "real" imaginary friend who tried to get Hurley to change, to rebel, to escape from the mental hospital. Later on, Dave re-appears on the island to try to convince Hurley that nothing he is experiencing is real; the island is fake; everything happening to him is a false - - - he is still back at the mental hospital. Dave tries to convince Hurley that he is dreaming by trying to convince him to jump off a cliff. Hurley is just about ready to jump, when Libby stops him.
Now, we learned that Libby was also a mental patient in the same day room as Hurley. However, on the island, Hurley never recognizes her. This is very strange considering that Hurley is extroverted when he is in the hospital. He feels safe and secure at Santa Rosa. This allows him to relax and let his mind wander. Why Libby does not introduce herself to Hurley as a fellow patient is mysterious. Does she have an ulterior motive to keep Hurley "on" the island? Is she a devil on one shoulder whispering in Hurley's head while Dave is on the other shoulder?
But even if Hurley was shy around women, he would have still known about Libby at Santa Rosa because the day room was small and open. And because of that fact, Libby would have been a memory, a character, in Hurley's own mind.
And Hurley's mind contains more fantasy than educational science. It appears that Hurley never went beyond high school. He was in dead end fast food career path. This would explain the "sci-fi" nature of the LOST universe, its inconsistent theories and scientific applications, because the source material is found inside Hurley's limited knowledge bank.
Hurley could have won the lottery in real life. He could have had the curse of the Numbers. He could have gone to Australia for answers. And when he got onto Flight 815 after his long trip, he would have seen all the passengers in their seats. It would have been stored in his short term memory. He had been reading a comic book about a polar bear on a tropical island. That also would have been stored in his short term memory. And when Hurley fell asleep during the long flight, he would use those elements to construct a fantasy tale about the island and his fellow passengers.
How can one cram six years of events into a 15 hour flight from Sydney to LA? It is easy because when a person dreams (REM sleep), a six minute dream may seem to last for hours. It is compressed imagery that the mind can process faster than in real life. But how can a person control his dreams to make complex stories? Researchers have found that some people can "set up" or manage their dreams by creating various stories, with or without themselves as main components. Other dreamers let their mind wander so they can experience new things or nightmares. Dream researchers believe that sleep is an important component to brain and human body functions. The rest period allows the physical body to recover from a day's work. The brain also needs time to "reboot" and organize itself. Many areas of the brain have less activity during sleep, while the creative side has an increase in activity.
Also, researchers believe that dreams play an important role in people's lives. Some believe that dreams help a person understand themselves. The dreams can be symbolic problem solving, taking real world issues and run various "solutions" that a person could remember and use in the future.
So it is possible that a person with an active imagination could dream a complex action-adventure in his sleep. And the clues that Hurley was being guided by Dave to free himself from the mental institution grasp parallel the grasp that the island had on him. Remember, Hurley is the last person on the island standing when the series ends. As such, Hurley could be considered the real man behind the curtain.
But that would be countered by the fact that the story ended in the sideways church, with Jack as the focal point. But, we do not know whether it was truly real. It could have been a reunion of Hurley's "imaginary friends" just as much as real friends.
There is a corollary to the Dream theory. I once proposed that it was not one person's dream, but a "collective, interactive" dream with all the 815 passengers. It would have been caused by the plane flying over the unique electromagnetic energy of the island, which could have altered the dream patterns of the sleeping passengers (since the human brain uses neurons and electric pulses to function). Under this theory, a person "dies" on the island (dream state) when he or she wakes up and is no longer connected to group. This also follows the unexplained concept of "awakening" in the sideways world. Awaken from what? People only awake from sleep - - - deep sleep or even day dreams where your subconscious takes over from your conscious state.
Then there is a third dream theory which seems strange. Instead of the whole story being in the mind of Hurley, it was all in the mind of the one survivor of the plane crash, Vincent. Vincent was the first "character" to move the story line forward in the pilot, and he was the last actor moving into position at the end.
Researchers believe that some animals do have dreams. Most land mammals experience the Rapid Eye Movement (REM) sleep where dreams mainly occur, but since they don’t keep dream journals, scientists tested rats to see what was going on in their brains when they slept. According to a 2001 report, MIT researchers Daniel Bedore and Matt Wilson placed trained rats on a track and monitored their brain activity while they moved towards their edible reward. They then monitored the rats’ brain activity while they were in a REM cycle. After examining the data, they saw that some activity in a sleeping rat’s brain matched some of its waking activity. The identical patterns led the scientists to believe that not only were the rats dreaming, they were dreaming about running on the track.
Dr. Stanley Coren, a psychology professor and neuropsychological researcher, writes in his book How Dogs Think: Understanding the Canine Mind, that dogs also dream. Like rats, dogs dream about common scenes they have experienced in their waking lives. Dr. Coren also notes that the smaller a dog is, the more it will dream: a small dog, such as a toy poodle, may dream once every ten minutes, while a dog as large as a mastiff or an Irish wolfhound may spend an hour and a half between each dream.
So, Vincent's laying down next to Jack at the end may have actually been the beginning after Vincent had surveyed the debris. Man's best friend trying to keep the dead passengers alive, at least in spirit.