Insomnia is one of the common sleep disorders. Despite of wide recognition that patients with insomnia subjectively experience impaired daytime functioning, there is little consistent objective evidence of neurobehavioural performance deficits. However, evidences of potential cognitive impairment in insomnia patients have been found in neuroimaging, neurobiochemistry and neuroelectrophysiology. This paper describes the characteristics of cognitive impairment in insomnia patients, then introduces the evidence of potential cognitive impairment found in neuroimaging, neurobiochemistry and neuroelectrophysiology, and finally analyzes the possible causes of this phenomenon.