Some science behind the scenes

Glucagon

Glucagon is a hormone secreted by alpha cells in the pancreas, that raises blood glucose levels. 

When energy is needed glucagon is released which activates an enzyme in the liver [phosphorylase] that converts the glycogen back into glucose ready for use. 

Its effect is opposite to that of insulin, which lowers blood glucose levels.The pancreas releases glucagon when blood sugar (glucose) levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream.

Thus, glucagon and insulin are part of a feedback system that keeps blood glucose levels at a stable level.