they dont. people can hallucinate while awake for many reasons. DMT is one of the chemicals used in dreams and out of body experiences.
when you are asleep, some parts of the brain shut down more than others. one of the parts that isnt working as much is the prefrontal cortex, which is what gives us logical thought. this is why in dreams we dont seem to care if crazy/unusual things happen.
if you look at a clock in a dream, then look away and look back, it will always be a different time. the brain makes it up as it goes.
