Way back, I took a contract flying three days a week to facilitate running a computer company that I owned. The pressure to learn the new technologies was huge, and I found the cruise sections of the flights very frustrating because I'd become used to soaking up knowledge for a large part of every day and the time seemed so wasted.
One time, and one time only, I took some computer stuff to read in the cruise. It immediately became obvious that I could not allow myself to become absorbed by another subject...my mind was pulled completely away from the job in moments. I slung the papers in my case and got on with being board. It's astonishing just how much you're taking in when you think there's nothing much going on.
It's funny, because one time I was having a planned snooze on a 4 engine piston aircraft, when an engine started making an odd noise.* My first officer said 'I don't know how you did that...you were asleep with your mouth open, and suddenly shouted for me to turn onto (Whatever heading).' Even asleep, I knew that the engine had a fault, and that I wanted to get my butt over land. I'm not sure that I would have been so attentive while being absorbed in another thing that I was passionate about.
So, the inference is that being distracted by something absorbing can be more of a problem than snoozing!
*(a valve head had busted off, and was rattling round on top of the piston.)