Pretty much, every martial arts class I ever attended took place at the end of my day. After work, when I was already physically and mentally tired. This is true for a lot of my students as well, whether young and still in school or older and engaged in the daily grind. It's easy for students, myself included, to lose focus and put forth less than maximum effort.
This is a huge mistake. Far too many students assume that if they ever need their martial arts skills they will respond like Jackie Chan, Bruce Lee, or Chuck Norris. I hate to have to tell you this, but on your best day you're probably not Jackie, Bruce, or Chuck.
We've all heard the phrase, practice makes perfect. I'm here to tell you it's not true, at least not in the sense that simple repetition will yield improved performance over time. Magicians the world over know that only perfect practice makes perfect. Practicing correctly, practicing with the intent to get better, practicing with the goal of improving each and every time; that is what makes perfect - or at least makes one better.
I tell my students all the time, "Make it real." Even when we're going through a progression of basics, you have to perform each block, strike, or kick as if this is the one technique that you need to save your life. You will perform no better in a real situation than you practice.
I recently, at the age of 50, began taking piano lessons. I've always wanted to play in the worst way, and now I do (har, har)! When the prospect of a recital was introduced I about panicked. Imagine when you were learning to dance, or fix a car, or sing, or play an instrument. Did it ever occur to you that when you needed to be at your best, maybe on a stage or getting paid, did it ever occur to you that you would suddenly sing, dance, solve a problem, or play your music better than when you had rehearsed it?
Don't you assume that under pressure you might make little mistakes, maybe so small only you would notice but mistakes nonetheless. Don't you pretty much know that if you are ever called on to do something that takes lots of practice that you'll never exceed the level of ability at which you've practiced?
Martial arts is the same. If you practice with detached interest, with limited focus, power, breathing, control, and intensity - that's pretty much how well you'll execute in the alley, parking lot, locker room, wooded lot, or bar and grille. However, if you focus during training, and concentrate on making each block, strike, or kick better than the last one; coordinating your breathing, and pushing yourself to improved performance - then, should you ever need to defend yourself, you will be effective without thinking.