Off hand, I just know when I'm playing a really tight/close game like BBC2, my pulse is *not at a normal resting rate. Plus, staying at a constant "full-alert" is taxing itself.

It's not the same as actual, physical activity, but it certainly isn't restful. I guess it's a bit like driving through really busy roads all day; at some point, your brain goes mushy and you aren't at 100%. You've been sitting down, but you're still worn ragged.