Book excerpt: GABA (gamma-aminobutyric acid) is the main neurotransmitter regulating sleep. The majority of drugs presently in use for the treatment of sleep disorders act by enhancing GABAergic neuronal inhibition. The GABA system is, therefore, of prime clinical relevance for the therapy of insomnia. The focus of this volume is on the neuropsychopharmacology and the clinical impact of the GABA system in regulating sleep and wakefulness. It presents molecular, neuropharmacological, systems-biological and clinical approaches to the understanding of the mechanism of action of GABA and GABAergic drugs. It also explores the role of GABA in the basic drives that affect sleep, and the influences that adapt sleep and wakefulness to external events.