The research reported in the paper is a part of a large project aiming
at designing an automatic device for the micro-sleep events detection. In the paper we are interested in the classification of EEG spectrograms with respect to the level of attention (mentation, relaxation, micro-sleep) of a monitored person (a proband). Data mining techniques are ušed for developing a classification model. Namely, GUHA method is employed for this purpose. It is a method of exploratory data analysis established on logical and statistical bases that has been continuously developed for last 40 years in the Czech Republic.