The sleep-promoting effect of isoliquiritigenin (ILTG) was investigated by analyzing the sleep architecture in mice. A hypnotic diazepam (DZP, 2 mg/kg) significantly decreased sleep latency by 39.7% and increased the amount of non-rapid eye movement sleep (NREMS) by 103.8% for the first 3 h after administration. ILTG (50 mg/kg) also produced a significant decrease in sleep latency (30.7%) and an increase in the amount of NREMS (61.1%). DZP significantly decreased delta (0.5-4 Hz) activity as compared with the vehicle; however, ILTG did not alter the delta activity. These results mean that ILTG induces sleep similar to physiological sleep without a decline in sleep quality.