Endurance and blood sugar

I've noticed feeling weak or fatigued more with low sugars than with high sugars, and I'm trying to figure out possible reasons for this.  I've already considered that insulin is in my system allowing sugar into my cells, but I still have too much sugar in my blood stream, and that fats can enter the cell without insulin.  Other than this I get confused as in my opinion in both extremes of blood sugar cells should be screaming for sugar.  Any thoughts?  Please try to give website or article references, this is something I want to be able to verify.

By low blood sugar I am assuming you are referencing sub 80's correct? Well my thoughts on this is not really that it is a lack of endurance it is the body zonking out due to lack of glucose in the system. I can tell you that if I am dropping I get sleepy and tired and sometimes have zonked out. It is just your body "slows" down. It happens more I notice when I run. I mean I will start out fine and go great and sure as I run I will eventually get tired. However, when my sugar drops again it is all I can do to get something into me, or I will just slowly go from a run, to jog, to nothing.

Again, I think it is more just being tired and run down which is how I feel without the proper amount of sugar in my system, which is why these things happen and why we are so concerned with low's.

Glucose is the fuel for your body's cells.  When you have a low blood sugar, your body feels like it is running on empty, which is why it slows down and you feel fatigued and weak.  Think of it this way, a car engine won't run without gas right?  Your body tries to conserve the glucose it has for your brain, so it sends signals to the rest of the body to slow down/shut down unnecessary processes including movement. 

The Merck Manual discusses the symptoms of hypoglycemia a little more and explains some why they occur: