Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The burden associated with participation is minimal. All blood samples, CAM-ICU scores and physical examinations required for the study are routine daily practice on the ICU. Adding clonidine for sedation of critically ill patients is common practice in many ICU's in the Netherlands.. Its use is also suggested in the NVIC guideline delirium on the ICU. It is however an off-label treatment. The major side effects of the study medication clonidine are hypertension, hypotension and bradycardia. Smaller studies have shown that these side effects are comparable to midazolam. Hypotension is a phenomenon that occurs very often in ICU patients, and is caused by different conditions, not only by the use of sedative medication. The benefit of participation is the possibility to reduce the period of delirium during ICU stay. Because of the widely off label use of clonidine in sedated and ventilated critically ill ICU patients this study is relevant to test the hypothesis that sedation with clonidine leads to a lower incidence and shorter duration of delirium.