Keep a regular sleep schedule.

This is something I completely failed to learn so far. Sure, I have some issues with procrastination or a lack of certain time-management skills, but even if I create a schedule for my whole week in advance and manage to follow it through for a couple of days at some point I completely mess it up because I sleep through half a day since I stayed up until 4AM the night before. Or I end up not getting enough sleep for several days in a row and getting sick (which happens far too often). Mostly, if I wake up at a certain time I don't get tired early enough to get a sufficient amount of sleep before I wake up at the same time on the next day (and unfortunately they don't make these time-turners yet).

It seems like every failed attempt to establish a working day routine can be mainly narrowed down to this single thing. I managed to get through High School and still get good grades even though I missed a lot of school days (due to being sick or too tired to go) because it was easy. Even at university it's still possible to pass the exams when you miss half of the lectures (although your results probably will suffer). However, I'm already afraid of my first real job.