When a part of the brain is in general use, an EEG picks up Beta waves. These waves tend to be irregular and low frequency, caused by milions of neurons firing at different times. However when the part of the brain goes to rest (it's not in active use), big huge Alpha waves are picked up. These waves come from a sumnation of neurons firing at the same time. It's like all of a sudden, all of these neurons are firing to a certain rhythm, at a very regular rate? How is this rhythm set? What's the purpose of this rhythm?