Just like in real life, your toon will get tired. The more you fight, the more fatigued you'll get. The small half circle under your toon's picture is the fatigue bar that you will need to keep an eye on while you are in battle. Now, this bar doesn't go down, it actually goes up the more fatigued that you are, so remember that the higher the bar, the worse you are. Your fatigue will raise when you get hit, when you fight too long, teleport too much, and more. What's even worse is that it will take forever for it to go back down again, since it decreases at a super slow pace. So, by simply keeping track of this bar and pacing your moves and damage taken, you can easily manage your toon's fatigue so that you aren't worried about it.