Delirium is a neurobehavioural syndrome that frequently develops in the postoperative and/or ICU setting. The incidence of elderly patients who develop delirium during hospital stay ranges from 11-82%. Delirium was first described more than half a century ago in the cardiac surgery population, where it was already discovered as a state that might be accompanied by serious complications such as prolonged ICU and hospital stay, reduced quality of life and increased mortality. Furthermore, the duration of delirium is associated with worse long-term cognitive function in the general ICU population. This long-term experience with delirium suggests a high socioeconomic liability and has been a focus of many studies. The aforementioned consequences of delirium are observed in all of three subtypes: hypoactive, hyperactive, and mixed. Pharmacological treatment options for hypoactive delirium are lacking. Since patients in hypoactive delirium suffer from disturbed circadian rhythm, the investigators suggest that the administration of melatonin as a promising possibility in these patients to shorten delirium duration and to lower its severity. Previous investigations confirmed loss of melatonin rhythm in patients that had developed delirium thus reasoning the study hypothesis. In this randomized study, the investigators aim to test the hypothesis that the reinstitution of a normal circadian rhythm by the administration of melatonin compared to placebo after diagnosis of hypoactive delirium, decreases the duration of delirium and reduces the transmission to a form of agitated delirium. The administration might have to be repeated several times to achieve resolution of delirium.