The brain consists of a hundred billion neurons, many of which might be firing off tiny electric zaps at a given moment. Putting a few electrodes on the surface of your skull won't tell you anything about whether specific brain cells are active, but it will give you a vague sense of how your brain cells are working together. When all the tiny zaps are measured in sum, they start to show up as broad patterns of activation that can be divided into different frequencies. The lowest frequencies, below about 3 hertz, are known as delta waves. The brain produces delta during the deepest stages of sleep. Theta waves aren't quite as slow as delta and reflect a drowsy state of mind. Alpha, which are a bit faster, correspond to relaxation and the absence of visual stimulation. High-frequency beta activity reflects alert behavior.