Monitoring glucose levels is central to all diabetics, and plays a key role in developing and maintaining a diabetic diet.
The human body breaks down carbohydrates into a fuel called glucose. When you eat carbohydrates the level raises, and as this fuel is used, levels are depleted.
Numerous other factors come into play though:
In the early morning the liver releases stores of glucose, this is called the dawn phenomenon, this is the initial energy that many people have after the nightly fast during sleep.
Stress hormones, such as cortisol, can cause glucose levels to rise.
Exercise can cause glucose levels to rise, similar to the dawn phenomenon, the liver can release glucose stores.
In a non-diabetic individual the pancreas would release insulin, regulating the blood glucose level, and helping that glucose metabolize, this is how insulin, and exercise (using that fuel) lowers blood sugar levels. In a diabetic individual, insulin is not produced, or not produced enough to regulate glucose levels, and metabolize the glucose, which is why exogenous insulin is required.