Brain waves are the different frequencies a brain can have. In short, the brain has a lower overall frequency when you are asleep, and a higher overall frequency when you are awake.

It has been found that the brain frequency called Alpha is very effective for creative thought and learning. The Alpha state is between the fully alert state and the sleep state. In the Alpha state you are relaxed and inward, more intuitive and creative. The highly alert state of the brain is called Beta. Our brain is in that state when we are engaging with the external world and dealing with things, people and events around us.

If an external frequency is applied to the brain, the brain is capable of changing its frequency. Applying external frequencies to the brain is called Entrainment. For example, if you apply a stimulus of 10Hz (this is within the Alpha range) to a person that is in the Beta stage (highly alert) for some time, the brain frequency is likely to change towards the applied stimulus, 10 Hz. The effect will be relaxing to the person. This phenomenon is also called frequency following response.

Entrainment works most efficiently when the external frequency is close to the brain's frequency at that moment.

The easiest way of applying stimulus to the brain is via the ears. However, humans cannot hear sounds low enough to be useful for brain stimulation, so special techniques must be used. One such special technique used is called binaural beats.

If the left ear is presented with a steady tone of 500Hz and the right ear a steady tone of 510Hz, these two tones combine in the brain. The difference, 10Hz, is perceived by the brain and is a very effective stimulus for brainwave entrainment. This 10Hz is formed entirely by the brain. When using stereo headphones, the left and right sounds do not mix together until they meet in your brain. The frequency difference, when perceived by brain this way, is called a binaural beat. For binaural beat you need headphones, so that different frequencies can be applied to the left and the right ear. There is also an entrainment method that uses the same frequency for both ears. This is called Monaural Beats. The Monaural Beat is a much simpler sound effect than the Binaural Beat effect. In the Binaural Beat sound, the brain creates the desired pulse frequency.

Note, however, that just passively listening to binaural beats does not necessarily alter your state of consciousness. For example, willingness and ability to relax and focus attention also determines how effective the binaural beat stimulus is for inducing state changes.

So, applying an outside frequency stimulus to the brain will eventually bring the brain's state closer to that frequency. This entrainment of the brain can be used to lower the frequency in the brain to stimulate a state of relaxation, focus, meditation and creativity. Some people have found that stimulating the Theta stage (4hz – 7Hz), which is even lower than the Alpha stage and is found in the brain in the dream state during sleep, for half an hour per day has reduced their need to sleep to just four hours a night. The theta stage has also been found to increase learning capabilities. In fact, children spend more time in theta stage than adults, which probably explains the accelerated learning capabilities of children.

When a certain brainwave state is experienced and practiced over a period of time, the brain will "learn" the state change and it will become easier to self-produce the desired brainwave state at will.

Brain wave entrainment is also used in treatment of depression, low self-esteem, attention deficit disorder, drug and alcohol addiction and autism, to name a few. It has also been found helpful in alleviating headaches and migraines.

Brainwave entrainment has had many historical, and often social, manifestations. Drum circles, chanting, and staring at a fire were all ways our ancestors engaged in this entranced state.

Modern methods include the development of tapes that induce certain frequencies in both ears. Uses of audio with embedded binaural beats are also mixed with music or various backgrounds. The applications of these tapes are different and range from relaxation, meditation, stress reduction, pain management, improved sleep quality, decrease in sleep requirements, super learning, enhanced creativity and intuition, remote viewing, telepathy, to inducing out-of-body experiences and lucid dreaming. Audio embedded with binaural beats is often combined with various meditation techniques, as well as positive affirmations and visualization.

A well-known source for these tapes, with 40 years of research into this field, is the Monroe Institute, with the Hemi-Sync products.