In my twenty-plus years of teaching, I can think of only a few students who were so intellectually deficient that they could not actually do an assignment. However, I have had a large number of students who had poor self-management skills. Just assume that it will be. Don't think that you can abuse your body and not pay the price. If you don't eat right, get enough rest, and exercise, you'll pay the price as the semester gets harder.
Illness negatively impacts grades.
Ditto mental stress. Normally, you'll make your worst grades those semesters when you had trouble at home, dorms, relationships, etc. Be gentle with yourself during those times. Ditto substance abuse. Myths aside, alcohol and dope won't make you a better student.