[smallerize]

Caffeine is a stimulant, acting on the sympatheitc nervous system. If you're also activating the parasympathetic nervous system, like by eating a meal, the effect won't be as strong.

Another effect of caffeine is as an adenosine re-uptake antagonist. Adenosine levels are usually low in the morning, so the effect is small. Later in the day, it can have a bigger effect on how alert you feel.