The goal of this clinical trial is to learn if a ketone drink can improve signs and symptoms of patients with a schizophrenia-spectrum disorder (SSD), or a bipolar-spectrum disorder (BD). The main questions it aims to answer are: Does a ketone drink improve information processing in patients with SSD/BD? Other questions it aims to answer are: Does a ketone drink improve cognitive functioning in patients with SSD/BD? Does a ketone drink improve metabolism and inflammation in patients with SSD/BD? Does a ketone drink affect circadian rhythm in patients with SSD/BD? Research will compare the effects of the ketone drink with that of an isocaloric carbohydrate drink in the same patients ('cross-over'). Participants will: 1. drink a ketone drink and (after a wash-out period) an isocaloric control drink (randomized order); after each drink: * EEG/EMG to determine information-processing parameters (PPI and P300) * cognitive tests * visual analog scale of mood, energy levels, ability to focus * indirect calorimetry to determine use of energy substrate * blood draws 2. for 5 consecutive days: * wear a continuous glucose monitor (CGM) * wear a non-invasive passive sweat biomarker sensor (EnLiSense device) * register a diet and nicotine diary * saliva sampling (max. 5x/day)