Why might someone fall asleep when on adderall (dextroamphetamine and racemic amphetamine)?

Dose strength?: If your dose is inadequate it will drop off before its ideal duration and you will experience a "let down" that can cause you to feel tired. Adderall (dextroamphetamine and racemic amphetamine) tablets should last 5.5 to 6-hours and Adderall (dextroamphetamine and racemic amphetamine) XR should deliver effects for 11 to 12-hours. If you do not have a substantial positive effect for that period, then you need an adjustment.