If you lived for just two months and had nothing to worry about but mating and eating, you would think your brain wouldn't get very tired. But the fruit fly's does, and the things scientists are learning about what happens when it sleeps can teach us about what happens when we do. Fruit flies doze off, even during the day, after engaging in intense social activity such as courtship, acclimating to a new environment or fighting over mates or territory. The insect's sleep cycles are regulated by three key genes; when scientists tamper with them, it stays awake. That causes its brain to get overloaded, since it misses out on neuronal pruning that goes on during sleep, as weak connections among brain cells are wiped out. This pruning creates the equivalent of disc space for another day. Similar things may happen as we sleep. By erasing the phone number you had to remember for only a day or the details of a movie you didn't like, you free your brain to learn better things tomorrow.