Studying is exhausting, and I'm not just imagining it:
When we sleep, the chemical energy reserves of astrocytes are replenished. This has been hinted at by experiment. Example: two groups (plus control groups) undergo intense physical exercise and intense mental exercise, respectively. Then they sleep. The group who exercised physically subsequently needs a normal amount of sleep (equivalent to the control group), while the group who underwent mental exercise needed more sleep. Thus we can conclude that neural activity (specifically heightened metabolic activity) is related to sleep.From Physiology of Behavior.