Thanks. That makes sense. The more you stand to lose by drifting off, the less likely you are to do it. Even if the cost of getting back on track is just pressing F11 or seeking back in the video, it can be a significant disincentive.

I think the whole computing experience -- sitting upright in front of a screen with a mouse in your palm -- puts your mind in "hunting mode". Web surfing is all about clicking, clicking, clicking. It's like a videogame -- a series of clicks produces a reward (or not): a funny video, a witty exchange, an interesting piece of news, an update from a friend.

I think this induces a certain state of videogame-style excitement, which is incompatible with "calm consumption", as when you're reading a book. On the Web, one minute you're playing "the Web game", the next you're supposed to read a serious text on math, history or geography. But you can't focus because your brain wants to get another dopamine fix. You need to calm down first, which takes time.