I've been looking into this recently. Specifically why drinking tea doesn't have the same jittery effects as coffee even though the caffeine content is similar.

And it all came down to the presence of L-Theanine in tea which is absent from coffee.

It's a remarkable amino-acid that influences GABA, dopamine and serotonin production.

Calm and focussed as opposed to running on high-adrenaline. Worth reading up on. :)