Modafinil, a eugeroic or wakefulness-promoting drug, is primarily used for treating narcolepsy, a sleep disorder characterized by excessive daytime sleepiness and sudden sleep attacks. Being a central nervous system (CNS) stimulant (psychostimulant) itself, modafinil has lower addictive potential than classical psychostimulants, such as amphetamines, cocaine or methylphenidate, but still produces psychoactive, euphoric, and subjective effects typical of classical stimulants.
Narcolepsy causes a strong urge to sleep during the day and can include symptoms like cataplexy (sudden muscle weakness), sleep paralysis (inability to move or speak while falling asleep or waking up), and hallucinations. Narcolepsy is linked to a lack of the brain chemical hypocretin (or orexin), primarily produced in the hypothalamus. Modafinil is not a cure for narcolepsy, but it can help manage the symptoms. While modafinil is primarily used to treat excessive sleepiness, it may also help reduce the frequency and severity of cataplexy attacks in some people. Modafinil is approved for management of narcolepsy with or without cataplexy. However, it is not specifically approved for the treatment of cataplexy.
Modafinil is also prescribed for shift work sleep disorder and excessive daytime sleepiness caused by obstructive sleep apnea, though it is recommended that people with apnea use continuous positive airway pressure (CPAP) therapy before starting modafinil.
Modafinil's use varies by region. In the US, it is approved for adult narcolepsy, shift work sleep disorder, and obstructive sleep apnea, but not for children. In the UK and the EU, since 2014, it is approved solely for narcolepsy, including in children (pediatric narcolepsy), with its use for other conditions restricted by the European Medicines Agency.
 both the French and the American Academy of Sleep Medicine strongly recommend modafinil as the first-choice treatment for narcolepsy. In Europe, modafinil is considered one of the primary drugs recommended for treating narcolepsy according to the guidelines.