There are four main types of EEG waves, classed by their frequency (technically, measured in hertz). Delta, the slowest wave, oscillates between one and four cycles per second, and occurs mainly during deep sleep; theta, the next slowest, can signify drowsiness; alpha occurs when we are doing little thinking and indicates relaxation; and beta, the fastest, accompanies thinking, alertness, or concentration. Gamma, the very fastest brain wave, occurs during moments when differing brain regions fire in harmony, like moments of insight when different elements of a mental puzzle “click” together.
