Today, the issue of preservation, maintenance and development of mental performance is of great importance due to social transformation and the school and higher education system upgrade. The growing body of information, continuous modernization of the training programmes, the use of various technical training aids that result in intensification of cognitive activity have a negative impact on the students' mental state. The study was aimed to assess the medical students' mental performance indicators in relation to the time of the day and the daily fluid intake. A total of 300 students were enrolled, who had both morning and afternoon (after lunch) classes. Mental performance of medical students was assessed using the Anfimov's table. It was found that the students' mental performance depended not only on the start time, but also on the water consumption regime. The main trends of water consumption in young adults were defined: moderate water consumption (1–2 L per day) prevailed, low water consumption (less than 1 L per day) was ranked second, and high water consumption (more than 2 L per day) was the rearest.