from what i understand very basically, and please feel free to correct me;
your blood carries about 3 minutes worth of Glucose but the brain needs a continuous supply.
The liver works constantly to keep up with the demand?
The liver can store around 12 hours of glucose, the main reason one should eat breakfast and include Carb's.
After an overnight fast the liver is deficient in stored glucose to parcel out for your brain, breakfast and a mid-morning snack does the trick.
The outcome is the feeling weak and or dizzy condition.
Hence my Carb's for building comment.