Why is it that when I am feeling real exhausted and sleepy I act like I’m drunk?

because your brain is trying to tell you it's time to shut down and sleep. it can only function properly for so long on so much sleep. it needs sleep to rejuvinate, so to speak. when it doesn't get it, it starts shutting down parts that aren't necessary to survival, like logical thinking. don't let yourself get to that point. it's not healthy. try to get at least 7 hours of sleep at night. it might be hard in your profession/life but it's important to your health.