Modafinil is a prescription medication primarily used to treat narcolepsy, sleep apnea, and shift work sleep disorder, conditions that cause excessive daytime sleepiness. Modafinil is typically prescribed in doses of 100mg to 200mg, taken once daily, usually in the morning to help improve daytime alertness. For shift work sleep disorder, it is often taken about 1 hour before the work shift. The medication can last for 8-12 hours, making it useful for managing daytime fatigue.