Scientists examined the glycogen contents of rats' brains at the rats' normal bedtimes and after they have been kept awake for an extra 6, 12, or 24 hours. The scientists found that glycogen was 38% lower for rats that had been sleep-deprived for 12 or more hours, and the levels recovered during subsequent sleep. These requirements speculate that we may need to sleep in order to restore the brains energy fuel.