Faster heartbeat is a side-effect from amphetamines, and if you're wide awake when you should be tired that's definitely an effect. Try doing something you enjoy. You should notice you enjoy it even more now. If time permits (you mentioned something that implies it's late where you live so I guess this ain't an option) go hang out with some friends. It's then that these subtle effects shine the most. You should be more talkative and social than usual.
Dexamphetamines' effects are pretty subtle in that dose range. It's a good dose for therapeutic reasons. But if you want a strong high I'd take more. Like start with 40mg (orally, screw sniffing all that binder) and then up my dose from there if I felt the need to.
Is it almost your bedtime? If so, try going to sleep. If you can't, they're working. Lol.
/e: another thing: I don't recommend taking doses over 60mg (atleast not until you're comfortable and familiar with the substance) since side-effects can get pretty bad at that point.