The influence of the benzodiazepine hypnotic lormetazepam (1 mg) and the cyclopyrrolone hypnotic zopiclone (7.5 mg) on heart rate activity was studied in 16 elderly insomniacs in a placebo-controlled, randomised, 3-fold crossover trial. After digital preprocessing of the ECG, QRS complexes were automatically recognised by a detection technique based on adaptative thresholds. Both R-R periodicity and heart rate variability were analysed as a function of sleep stages and time of night. Under placebo, heart rate decreased significantly from the first to the second half of the night. The relationship between sleep stages and heart rate remained constant under both hypnotics. Although the two substances significantly modified the distribution of sleep stages, no relevant changes in ECG activity were observed when the proportion of the different sleep stages was taken into consideration.