Hi there, it's Matt here and welcome back to the podcast.

Today I want to tell you about a piece of physics discovered in 1838 by a Prussian meteorologist

who almost certainly never imagined it would end up as one of the most downloaded audio categories on Spotify.

Picture this. It is sometime around midnight. Someone, let's call her Eleanor, is lying in bed with her phone

balanced on her chest and a pair of over-ear headphones clamped to her head. She has been awake

for 40 minutes, not anxious exactly,

not in pain, just awake in that specific maddening way that every person reading or listening to

this will recognize immediately. Her mind won't land, it keeps circling. And so she opens an app,

scrolls past ocean waves and rain on

tent, and lands on something called delta binoral beats, deep sleep induction. She presses play.

She has no idea what a binaural beat is. She has no idea whether any of this will work. She only knows that she is

tired and that the internet has told her this might help. The question I want to spend this

episode answering is, what does the science actually say? There is a real story here. And like

most good stories in sleep science, it starts with something

unexpected. It starts with tuning forks. Let me begin with a simple question that almost nobody

who uses binaural beats can answer. What precisely is one? Think of the last time you watched a helicopter pass

overhead. As it approaches, the rotors are cutting through air at a certain speed. As it moves away,

the sound shifts. Lower, slower. What you are perceiving is not just the helicopter itself, but the relationship between

the helicopter's frequency and your ears. Now shrink that entire phenomenon down to something happening,

not between you and an aircraft, but between two invisible tones and your brainstem. That is something

close to what a binaural beat is, except the sound you hear isn't physically real at all.

Here is the exact mechanism you put on stereo headphones. Into your left ear goes a tone of, say, 200 hertz, into your right ear goes a tone of

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