Sleep is vital to maintain health and well-being; however, insomnia is currently a widespread health complaint worldwide. In particular, caffeine, a psychoactive component of coffee, tea, and caffeine beverages may lead to sleep disorders such as insomnia. In this study, our primary objective was to investigate the inhibitory effect of high-purity phlorotannin preparation (HP-PRT) on caffeine-induced wakefulness. The sleep test of pentobarbital-induced mice was used as an in vivo animal model. Caffeine (50 and 100 mg/kg) showed significant arousal effects (an increase in sleep latency and a decrease in sleep duration). Co-administration of caffeine (50 mg/kg) and the sedative-hypnotic diazepam (DZP, 1 mg/kg) did not result in similar arousal activity. HP-PRT (500 mg/kg) also inhibited caffeine-induced wakefulness. Our results suggest that HP-PRT would be a useful additive for developing coffee products without the arousal effect.