"Can I eat carbs sometimes? Can I smoke and drink occasionally? Can I play video games or waste my life in other ways?"

Actually, you can, of course. Everything depends on your goals.

Can you smoke several cigarettes a day? Yes. Can you drink alcohol occasionally? Yes. And you will be ok.

But if you strive for excellence, you do whatever you can to be better. Ultimate success requires ultimate efficiency. Or you can always remain mediocre...