Caffeine is a mild stimulant, but in the evening the main effect is probably blocking your adenosine reuptake sites. https://en.wikipedia.org/wiki/Caffeine#Pharmacology You can probably get a similar effect by drinking a small amount of caffeine in the early afternoon instead of a large amount in the morning. I mentioned this in another comment, but slowly decreasing your caffeine intake over time lets your body adjust with pretty minimal symptoms.