I always think about this. I was a pretty lazy teen, but after I got mono I reached a whole new level. Nearly failed my first semester of university, slept until 10-11 and had to start building class schedules around that (despite waking up at 630 to get ready every morning and arriving to high school an hour early), went from working out three days a week to feeling too tired to cook lunch (which didn't help the spiral). There are lots of possibilities - new environment, more difficult class level, less supervision, but I never felt so "out of it" in my life as that first two years.