Daily rhythm of serum melatonin levels and effect of light exposure in patients with dementia of the Alzheimer’s type

Background: Several studies have suggested that patients with dementia experience a deterioration of biological rhythms. We investigated the daily profile of serum melatonin levels in patients with dementia of the Alzheimer’s type (AD), since daily melatonin rhythm is thought to reflect the functioning of the biological clock. Methods: Seventeen inpatients with AD, 10 psychiatric inpatients without dementia, and 11 elderly healthy volunteers participated in this study. Serum melatonin was assessed every 3 hours by radioimmunoassay. Results: A daily fluctuation of melatonin levels with a significant nocturnal increase was observed in all three subject groups. However, both the AD patients and psychiatric patients without dementia showed significantly higher levels of melatonin in the daytime compared with the healthy subjects. When the effect of bright light exposure on melatonin secretion was investigated in six AD patients and five psychiatric patients without dementia, the daytime levels were markedly decreased in the patients without dementia, while no change was observed in the AD patients. Conclusions: The high levels of melatonin in the daytime associated with a lack of response to light exposure in AD patients may be due to the neurodegenerative process of this disease.