I suspect it's similar to my experience with many video-lectures: on one hand, they are almost always filled with enough new information to keep my attention. However, the pace at which this information is fed to me can be just a bit too slow, meaning that my brain has to do too little work to process it. This combination of keeping attention while not activating my own thinking has an ability to put me sleep in a very drastic fashion. As in, trying to pay attention to the lecture and literally falling asleep in the blink of an eye.

The only way to counter this is to download the lectures and play them back at an increased pace - 1.2x to 1.5x, depending on the difficulty of the subject.

Perhaps the novels have the same issue for Kenji: too little stimulation to keep the mind going, even if it keeps your attention.