For me it's not falling asleep, but classically getting lost in thoughts. When I'm on medication I think about random things too, but automatically check the street every half a second consciously. If not, it often happens that I suddenly realize the car in front of me has been getting a lot closer since the last time I paid attention to it, which is before I got completely lost with my full mind in a subject. It's also a lot harder to turn and shift smoothly without meds, as I then do it automatically without even being able to concentrate on it.