My understanding is that the maximum amount of caffeine is extracted from tea between the 3.25 and 3.75 minute mark of the brewing cycle. You get a smaller amount of caffeine brewing it longer than that, but its effectiveness in the bloodstream is offset by the increased amount of tannin and other plant esters that are also being extracted. Tannin, by itself, will affect the heart. It causes a jittery sensation at higher (i.e. mildly toxic) concentrations, but it does not provide anything in the way of an energy boost.
You do not get less of anything, the longer you brew your tea...you get more.
Commercially produced decaffinated tea is usually made by running the tea through hot water for 20 seconds and then drying it, before packaging it... so I guess that would mean most of the caffiene would be released in the first 20 seconds, otherwise they couldn't call it "decaffinated", right?
There is something else in tea besides caffiene, that its side effects are closely related, and can cause your heart to race and give you jitters (and make kids bounce off the walls for hours)...namely theophylline. And it's effects are longer lasting than caffiene, lasting about 5-8 hours, with smokers metabolizing it faster than non-smokers.
This chemical traditionally was used to treat asthma, and the first pharmaceutical grade prescription stuff actually came from tea & coffee (mostly tea). Have you never heard of giving someone that has asthma a cup of strong tea or coffee, in an emergency? This is why.
But the bad side effects of this drug make it not such a good choice today, especially when there are other things that work better without the negative side effects and dangers that theophylline has.
While tea may have less caffiene than coffee, it has higher amounts of theophylline than coffee, and that is released later in the brewing process. So the longer you brew your tea, the more theophylline you'll have in it...but not at the level that you'd get from prescription strength theophylline pills.
But if you are a heavy tea drinker and brew it double strength with 2 tea bags instead of one, and use a long brewing time, you are going to get a lot of both caffiene and theophylline.
That's the double bang with tea. When the caffiene has worn off, the theophylline is likely still having an effect on you.