It's probably easier to overdose on caffeine pills or on espresso than on modafinil. It helps with mental focus as well even if you're well rested and had adequate sleep. It is similar to adderall in this aspect (though it does not stimulate as indiscriminately, you don't get elevated heart rate, etc.) However, it has a very long half life and it can mess up your sleep patterns. More importantly, it is considered a doping and therefore prohibited in competitions (as far as I know, for no good reason - it has not been shown to improve your performance), and it is a controlled substance in the U.S.