The goal of this clinical trial is to investigate the effectiveness of standardized and personalized transcranial alternating current stimulation (tACS) in patients with delirium. To investigate this, will perform a double-blind, randomized, sham-controlled, multicenter trial. To test the safety and feasibility of tACS in delirious patients, the first 30 patients will be randomized to receive either active or sham tACS treatment in a 1:1 ratio through a pilot study. For the main phase of the study, patients will be randomized, resulting in an overall 1:1:1 allocation to standardized tACS, personalized tACS or sham treatment. Electroencephalogram (EEG) will be measured directly before and directly after the first stimulation session to assess whether tACS can reduce relative delta power. tACS or sham treatment will be given once daily for a maximum of 14 days, or until resolution of delirium or discharge (whichever comes first). During the treatment phase, presence and severity of delirium will be assessed daily. After the treatment phase, an additional EEG will be measured. Cognitive status will be assessed three months after the initial tACS session.