About 20% of adults experience excessive daytime sleepiness or severe fatigue. Causes include somatic conditions, psychiatric disorders, and medication or drug use. Treatment depends on the underlying cause. If sleepiness persists despite optimal treatment of the underlying condition, exclusion of other causes, and behavioral interventions, wakefulness-promoting agents may be considered. However, no established pharmacological strategy exists for symptomatic treatment. Modafinil and stimulants like methylphenidate may offer some benefit based on experiences with narcolepsy or idiopathic hypersomnia. Studies in specific patient groups (e.g., multiple sclerosis, Parkinson's disease, traumatic brain injury, cancer-related fatigue) show variable results. The use of wakefulness-promoting agents is discouraged for addressing unexplained fatigue, as seen in the context of chronic fatigue syndrome.