Fatal familial insomnia, Morvan's chorea and delirium tremens share the same clinical features: severe insomnia and mental confusion with dream enactment, associated with motor and autonomic activation. Polygraphically, they share an inability to generate slow wave sleep. Agrypnia excitata is the term which aptly defines this peculiar medical condition. In fatal familial insomnia, the syndrome is due to a functional imbalance between activating and deactivating structures within the limbic system provoked by the atrophy of the mediodorsal and anteroventral thalamic nuclei. In Morvan's chorea and delirium tremens, a functional imbalance within the thalamolimbic circuits might be explained by the accumulation of some antireceptor antibodies and by a transient prevalence of excitatory over inhibitory synapses, down-regulated by chronic alcohol abuse, respectively. The selective disappearance of slow sleep (i.e. sleep spindles and delta rhythms) characterizing the agrypnia excitata syndrome, together with other clinical and experimental findings, suggests that sleep can be divided into three types. The most archaic form of sleep corresponding to stage 1 non-REM sleep is shared by man and poikilothermic animals and generated within activating and deactivating neuronal poles located in the basal forebrain, hypothalamus and brain stem; the other two forms of sleep, slow wave sleep and paradoxical sleep, confined to homeothermic animals, are generated in the thalamus and pontine reticular formation respectively. 2001 Harcourt Publishers Ltd