Humans need constant sensory stimulation. Sensory information helps us orient our place in the world and alert us to danger, novelty, and ways to survive. A complete lack of sensory input is abnormal, and as humans we can find it hard to cope without this information.
For example, an article published on smartplanet.com describes the “Quietest place on earth’, a room designed for laboratory testing that absorbs 99.99 percent of all sounds (“‘Quietest place on earth’ causes hallucinations”, Tuan C. Nguyen). When humans even spend a short amount of time in this kind of environment, they are able to hear things they don’t normally notice, like their heart beating, and sometimes the brain may even hallucinate to make up for the lack of sensory input. For example, visual deprivation can result in hallucinations in healthy people (read a case study here).
The brain is always looking for new and changing information. Related to sensory deprivation is something called the Ganzfeld effect, which is what happens in response to a prolonged exposure to a uniform, unchanging stimulus, for example, an undifferentiated field of the same colour. Rather than sensory deprivation, this is called perception deprivation (because you are seeing something, but that thing doesn’t change). Sometimes when people are exposed to this, they hallucinate, and researchers have even found that people’s brain activity patterns change in response to this type of stimulus (read about this type of research in this review article published in Cortex in 2008).
It’s a very interesting area of research if you are interested in why humans hallucinate. One reason could be that the brain is increasing alertness and perception in order to find something new, anything at all, in response to a lack of sensory input. However, I think it’s important to note that this might be a very different reason than the reason that people with psychosis hallucinate.