Not to take anything away from Tyler1, because he really goes above and beyond, but it really doesn't take much mental focus or anything really to play it. Programming? It takes focus, you really have to think sometimes. But League? When I'm tired the game plays itself. It just feels like muscle memory.
It might take a bit more effort to play at a consistently high level. But I'm pretty sure it's mostly muscle memory for him too. There's very little actual thinking other than the big picture of what objectives to take and I'm sure when he's playing for 20+ hours he's just on autopilot for the majority of the time.
To put it into comparison with how little mental power it personally takes me, I like to watch a lot of Anime, that's Japanese animation with English subtitles. I can't watch Anime if I'm tired because keeping track of subtitles and what's on screen actually takes a substantial amount of mental power. But I can bash out 5 games of LoL back to back when I'm equally as tired.