In the present study, we attempted to characterize the circadian rhythm of locomotor activity of El mice and to examine the duration of sodium pentobarbital-induced sleep during the light and dark periods. The effect of Saiko-ka-ryukotsu-borei-to, a representative Kampo medicine used for treating insomnia, was studied on the locomotor activity and on the duration of sodium pentobarbital-induced sleep in El mice. The spontaneous locomotor activity of El mice during the dark period was not so different from that of ddY mice, whereas the activity during the light period was significantly higher in the El mice, and the duration of sodium pentobarbital-induced sleep of El mice was very short during the light period, nearly equal to that during the dark period. The administration of Saiko-ka-ryukotsu-borei-to caused marked reduction in the locomotor activity during the light period, and dose-dependent prolongation in the duration of sodium pentobarbital-induced sleep time during the light period, but that during the dark period was unaffected. These findings suggest that the El mouse is a model with a tendency to be easily excitable during the light period compared to ddY mice. Furthermore, it was confirmed that Saiko-ka-ryukotsu-borei-to reduced the excitation in El mice and might be useful against sleep disorder due to excitation.