The fundamental conception of delirium is altered arousal. In addition, sleep-wake cycle disturbances including insomnia, excessive daytime napping, and disintegration of the expected circadian patterns have been described as a characteristic component of delirium for decades, and demonstrated to be a core symptom domain of delirium. Although non-pharmacological interventions are successful to some extent, they have limitations due to various biological etiologies for delirium. Among pharmacological interventions, antipsychotics seem to be effective, but they are not suitable for preventive use because of relatively frequent side-effects such as extrapyramidal symptoms. Recently, new type of drugs for insomnia have been focused with respect to delirium prevention. Recent meta-analyses show effectiveness of melatonin receptor agonists and orexin receptor antagonists for delirium prevention, and real-world data support them.