During sleep, the body provides fuel to the brain during sleep, the body provides fuel to the brain by breaking down body proteins. by converting glucose to glycogen. the body does not provide fuel to the brain during sleep. by engaging in ketosis

The answer to this question would be: by converting glucose to glycogen. 

Brain cells only able to use energy from glucose, so if the body glucose level is too low the brain cells can't work as it doesn't get any energy/food. When glucose level is decreased, the liver will start to convert glycogen into glucose to keep the glucose level.