Modafinil has a half life of 15 hours, how the hell does anyone get any sleep after taking it repeated days in a row - never mind twice a day for a 4 years, at

Modafinil has a half life of 15 hours, how the hell does anyone get any sleep after taking it repeated days in a row - never mind twice a day for a 4 years, at the end of that he was on ridiculous amounts.