Insomnia is a common symptom in schizophrenia, although it is seldom the predominant complaint. Sleep-onset and maintenance insomnia is a characteristic feature of schizophrenic patients regardless of either their medication status (drug-naive or previously treated) or the phase of the clinical course (acute or chronic). Regarding sleep architecture, the majority of studies indicate that stage 4 sleep and rapid eye movement (REM) latency are reduced in schizophrenia, whereas REM sleep duration tends to remain unchanged. Insomnia in schizophrenic patients could be partly related to the presumed over-activity of the dopaminergic system. However, there is a possibility that the GABAergic system is also involved in sleep disturbance in schizophrenia. Since many signal transmission systems within the CNS can be implicated in the reduction of REM latency in schizophrenia, the characterization of the neurotransmitter systems involved remains a challenging dilemma.