Glucagon is the one responsible for letting the liver know when it is time to convert glycogen into the much-needed glucose. Glucagon will become active when the blood sugar levels drop. Basically, what glucagon can do is to make the liver increase the blood sugar levels of the body.
To make sure that it will not be too high, insulin will lower the blood sugar levels until this is completely neutral. When the blood sugar level is too high, this is already a sign that the person is diabetic. Some would even need to take insulin shots just to regulate the blood sugar levels of the body.