Oneirophrenia is a mental state which is characterized by hallucinations. These can be induced by lack of sleep, sensory deprivation, or psychoactive drugs.
Sleep deprivation
Lack of sleep is something I am quite familiar with as a trucker. In my early career I've gone for as much as a week on less than 20 hours of sleep. My routine was to arise on Sunday morning, do whatever activity during the day, then roll out on Sunday evening. The first 2 days were easy enough. By the time Tuesday morning rolled around I'd have to get about 2 solid hours sleep. The rest of the week was similar, grabbing a nap just before sunset until it was dark, then another short one about dawn. I'd generally arrive home Saturday morning, totally wiped out. I'd squeeze in whatever rest and recuperation I could, then roll out again Sunday evening. I've maintained this schedule for months at a time.
Get sleepy enough and you'll start seeing things. I've seen my friend's car stopped in the road in front of me, went on the brakes only for the illusion to disappear. I've driven through Christmas trees growing on the roadway, had huge hawks land on my hood, seen the red sunrise break and run down the mountain sides like Kool-Aid. All the while my body cried out, exhausted, running on empty. Going without adequate sleep is like writing bad checks: your bank might let you bounce a couple small ones, but at some point you will pay.
Sensory deprivation
Sensory deprivation is another cause of a hallucinatory state. Our minds are constantly bombarded with stimuli, much of it unconscious. We are designed to deal with this torrent of information. When we are deprived of this flow, our minds will sometimes provide its own stimulation. We could experience conversations with persons long dead, see visions, or experience any of a pallette of non-reality based stimuli.
Drugs
Drugs of course are the method of choice for many seeking an excursion into an alternative reality. LSD, PCP, METH, pot, name your poison. Drug induced trips have the inherent danger of the ticket being one way, no return travel guaranteed. Sure, most trips are interesting and end without harm to self or others. There are the exceptional journeys which end in severe consequences.
It is not my intent to preach against drug usage. My philosophy is to make reasonable choices and accept the result of those choices. I always considered myself marginally stable sans drugs, so any experiments would probably end badly for the hometown team, so to speak.
Sources:
http://www.answers.com/topic/oneirophrenia?cat=health