Most organisms are in a constant state of "burning fuel." For example, even in a resting state, our bodies "burn calories" to keep our heart beating, our breathing constant, our digestion and even brain function going. Less activity requires less fuel and this is why some organisms go into states of hibernation or suspended animation during times when fuel sources are unavailable (such as bears in winter). In this hibernation state, the body functions slow down to a bare minimum to sustain life, allowing the organism to use the least amount of fuel possible. Without a consistent source of fuel, life cannot be sustained.