Complex neurobiological models based on animal research have been formulated in an attempt to explain the cyclic pattern of nonREM and REM sleep. The "reciprocal interaction model" of nonREM and REM sleep regulation, which has been updated to incorporate new evidence is still the most convincing. Therefore it is reasonable to apply this model also to REM sleep abnormalities such as shortened REM latency and increased REM density, observed in patients with depression and alcohol dependency. In a retrospective analysis baseline data from 40 subjects with primary alcohol dependency are compared with a group of 40 patients diagnosed with major depression (diagnoses according to DSM-III-R) and healthy subjects. All alcohol dependent patients were examined in the sleep laboratory during subacute withdrawal at least 7 days off medication and after at least 14 days of abstinence. The patients with major depression (at least 7 days off psychoactive medication) and the healthy subjects had been examined previously by polysomnography during the last few years in the context of various studies and were assembled from our database to match the group of alcohol dependent patients with respect to age and sex. Alcohol dependent patients exhibited similar disturbances in sleep continuity and REM sleep as depressed patients in comparison to healthy controls while parameters of sleep architecture were even more strongly disturbed in alcohol dependence. While enhanced sensitivity of cholinergic receptors is the most likely explanation for the increase in "REM pressure" in depressives, this appears not to apply to alcoholics, who rather exhibit a decreased response to cholinergic stimulation. Thus, according to the reciprocal interaction model of nonREM- and REM sleep regulation and in contrast to the interpretation of the findings in depressed patients, an impaired aminergic rather than an increased cholinergic neurotransmission might be responsible for the increased REM sleep pressure in alcohol dependent patients. Alternatively or in addition the REM anomalies in alcoholic patients could also be due to adaptive regulatory processes during chronic alcohol consumption that lead to downregulation of GABA(A)- and upregulation of NMDA-receptors or their intracellular signalling and become apparent with alcohol withdrawal. Such adaptive counterregulation might also explain the alterations in slow wave sleep found in alcoholics that are even more pronounced in these patients than in patients with major depression.