Binaural beats are an auditory phenomenon in which two tones of slightly different frequencies are played in separate ears simultaneously, usually via headphones. When exposed to both tones at once, the human brain perceives the creation of a new, third tone, whose frequency is equivalent to the difference between the two tones being played.
Some evidence suggests that the tone being perceived can alter brain waves through a process called entrainment, in which brain waves align themselves to an outside beat or frequency. Since different brain wave patterns are associated with certain states of mind, from relaxed to anxious to productive, binaural beats have been theorized to impact mood, focus, and sleep by altering brain waves.
As a result, the phenomenon has gained popularity in recent years as a potential therapy for anxiety, insomnia, concentration difficulties, and more. Though evidence for the efficacy of binaural beats remains preliminary, mixed, and based mainly on small studies, most experts agree that the risk of side effects is low, and many apps, podcasts, or other online services provide free access to binaural beats—often in conjunction with guided meditations, music, or other auditory stimuli intended to promote relaxation or focus.