Abstract Covering sleep research, normal sleep, and then sleep–wake disorders, this chapter splits abnormal sleep into insomnia, hypersomnia, circadian rhythm disorders, and parasomnias. Individual cases, such as sleep-related epilepsy, panic attacks, and sleep-related violence, are all defined. The effects of psychiatric medication on sleep (either sedative or alerting) are defined, as well as the effects of recreational drugs. The intersection of other psychiatric disorders on the sleep schedule are described, and potential differential diagnoses listed.