Neuronal histamine is presumed to play a major role in regulating diurnal fluctuations such as sleep-wake rhythm. There were, however, no data showing a biological rhythm of neuronal histamine in human health or in neurodegenerative disorders. In postmortem human brain material obtained from the Netherlands Brain Bank, we determined with quantitative in situ hybridization the mRNA expression of histidine decarboxylase, the key enzyme of histamine production, in the tuberomammillary nucleus of the hypothalamus at different time points of the day. With nonlinear regression periodic mathematical model we revealed a diurnal fluctuation of neuronal histamine production in control subjects, which was disturbed in neurodegenerative disorders (Parkinson’s disease, Alzheimer’s disease, and Huntington’s disease). These findings offer a basis for further study of the role of histamine in the signs and symptoms of neurodegenerative disorders such as sleep disorder. The paper was published in Sleep.