Focused later in the day after taking meds in the morning/early afternoon ??!

I tried a second smaller dose of Elvanse today (30mg in the morning 15mg at 1-2pm). Why do I feel more clear head and focused at ) 6pm than I did earlier in the day? Is that normal? I am aware the 15mg is to carry me through as I was getting a crash early from the initial dose and it should still be effective. However earlier in the day I was a little bit agitated. I’m a bit confused :s