The following images, taken from the excellent Mapping the Mind (Rita Carter, Weidenfeld and Nicolson, 1998) highlight the activity within the brain under various states of mind.
Vivid visual dreams light up the visual cortex; nightmares trigger activity in the amygdala and the hippocampus flares up from time to time to replay recent events. The areas which seem to be most commonly active are the pathways carrying alerting signals from the brainstem and the auditory cortex; supplementary motor area and visual association areas -- all of which produce the 'virtual reality' effect of dreaming.
Meditation: Scans of people in a self-induced state of 'passive attention' have been shown to 'turn off' areas of the brain normally associated with seeking stimuli, including the parietal, anterior and premotor cortexes.
Lack of activity in the frontal lobes is a feature of states of mind in which consciousness is disturbed or decreased. In chronic schizophrenia the dorsolateral pre-frontal cortex is especially hypoactive. This might account for the state's common reduction in planned or spontaneous behaviour and social withdrawal. the anterior cingulate cortex -- thought to distinguish between external and internal stimuli -- is also underactive, which may be one reason why schizophrenics confuse their own thoughts with outside voices.
In slow-wave sleep the entire brain oscillates in a gentle rhythm quite unlike the fragmented oscillations of normal consciousness. Brain scans show less activity in the limbic system.
(p18) Mapping the Mind Rita Carter. Publ: Weidenfeld and Nicolson, 1998