Modafinil is a central nervous system stimulant used to treat excessive daytime sleepiness from conditions like shift work disorder, narcolepsy, and sleep apnea. It works by inhibiting dopamine reuptake, increasing dopamine levels and stimulating other neurotransmitters like norepinephrine and serotonin. Common side effects are mild and include headache, nausea, and decreased appetite in under 10% of users. While generally well tolerated, it should be discontinued if serious rashes develop.