Excellent point. The workout analogy is good. When I'm really studying something, I eventually get a 'heavy' or 'warm' feeling in my body from the effort of concentration. You can feel how metabolism increases from the sustained effort.

I think those light videos can work as an introduction, but then you have to start to dig in. Watch something for 20 minutes and make notes. Then study 2 hours with 1 to 2 breaks. I can't keep studying more than 2 hours a day because it's exhausting, but I may do a 30-minute refresh and recap later in the same day.