Brainwave synchronization or "brainwave entrainment," is any practice that aims to cause brainwave frequency to fall into step with a periodic stimulus having a frequency corresponding to the intended brain-state (for example, to induce sleep). It depends upon a "frequency following" response, a naturally occurring phenomenon where the human brain has a tendency to change its dominant EEG frequency towards the frequency of a dominant external stimulus. Such a stimulus may be aural, as in the case of binaural beats, monaural beats, and isochronic tones, or else visual, as with a dreamachine, a combination of the two with a mind machine, or even electromagnetic radiation.
