I'm diabetic and I know how important it is for the brain to get glucose regularly. Sometimes my medication doesn't quite kick in and even though there is sugar in my blood stream, the glucose doesn't reach my brain because my insulin doesn't work.
I always recognize the signs. I become confused, dizzy and extremely tired. These are tell-tale signs of sugar not reaching the brain. Thankfully, I've always recovered in about half hour to hour. But it always feels like I've been run over by a truck afterward and I have to rest for the remainder of the day. It can be quite debilitating.