Modafinil is a wakefulness-promoting medication that is used to treat excessive daytime sleepiness associated with certain medical conditions. The three medical disorders for which modafinil is commonly prescribed are:

Narcolepsy: a neurological disorder that causes excessive daytime sleepiness and sudden sleep attacks.
Obstructive sleep apnea/hypopnea syndrome (OSAHS): a condition in which the upper airway becomes blocked during sleep, causing interrupted breathing and poor sleep quality, which leads to daytime sleepiness.
Shift work sleep disorder (SWSD): a sleep disorder that affects people who work night shifts or rotating shifts, causing them to feel sleepy or fatigued during work hours and struggle to stay awake during their normal sleep time.

It's important to note that modafinil is a prescription medication and should only be used under the guidance of a healthcare professional.