In the medical literature, idiopathic hypersomnia may also be referred to as IH, IHS, primary hypersomnia, central hypersomnia, or hypersomnia of brain origin. The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) defines idiopathic hypersomnia as EDS without narcolepsy or the associated features of other sleep disorders.[7] It occurs in the absence of medical problems or sleep disruptions, such as sleep apnea, that can cause secondary hypersomnia, and is therefore considered a subset of the primary hypersomnias, which 'are thought to arise from problems with the brain’s systems that regulate sleep and wake.'[8]