The sedative or excitatory effects of drugs are difficult to evaluate in patients with depression, where sleep disturbances and tiredness in the daytime belong to the clinical manifestations of the psychiatric disorder. A refined method of vigilance measurement, based on the EEG spectra, together with proper statistical analysis of the data, is helpful for correct interpretation of the data. In two groups of patients with depression, the intensity of sleep disturbances was considered as a background variable in partial correlations, reflecting the relationships between vigilance and drug concentration in a more specific way. It was shown that the sedative effect of maprotiline interferes with the increased vigilance in the patients, with improved night sleep after treatment. As a result, the patients do not experience decreased vigilance although maprotiline has a sedative action. The results obtained in the patients treated with β-blockers suggest that the drug itself has no sedative effect but the patients suffer from decreased vigilance in the daytime, caused by the sleep disturbances and depression