Good observation, Kptclutch...
The ADD brain is very fast, but quickly wears down on a normal day. Anti-histamines make you drowsy, some more than others, but not taking them should help.

Exercise also helps a lot. On days when I get my two miles in the morning before work II feel so much better than days when I don't. When I get my evening two miles in, I eat less and get sleepy more easily. It is the dopamine released by the brain during exercise that makes the difference.