To my own experience, I think the time an adult can physiologically commit to learning is reduced. For example, I noticed that in high school if I really liked

To my own experience, I think the time an adult can physiologically commit to learning is reduced. For example, I noticed that in high school if I really liked a subject I would just sponge it all up, no need for spaced repetition and I could be focused for hours. In college, I started to feel the need for spaced repetitions but yet be focused for hours. Now, if I get to do something I really enjoy intellectually, I can get 30-45 minutes of focus. Then rest and spaced repetition. Last time I tried to push 3 hours of study, I not just forget most of the content but it affected my mood so badly. Somewhat, I see the same pattern with physical activities. I could run have hard workouts back to back in high school while now it is so easy to overtrain and rock my hormones that I need to be really careful.