From brainwave bands to sleep onset — a plain-English tour of the sensor resting on your forehead, and how it hears the exact minute you fall asleep.
What an EEG actually reads
An electroencephalogram, or EEG, doesn’t read your thoughts. It reads voltage. Billions of neurons firing together produce tiny electrical rhythms at the surface of your scalp — measured in millionths of a volt. A dry sensor pressed to the forehead picks up those rhythms and turns them into a live waveform. That’s the same signal sleep labs have used for decades, now small enough to sit inside the band of the Recharge™ Mask.
The four bands, decoded
Your brain doesn’t run at one speed. It shifts through frequency bands as you relax and descend into sleep:
- Beta (13–30 Hz) — awake, alert, mid-conversation or mid-email.
- Alpha (8–12 Hz) — eyes closed, calm, the doorway to sleep.
- Theta (4–7 Hz) — light sleep; you’ve drifted off but wake easily.
- Delta (0.5–4 Hz) — deep, slow-wave sleep, the most restorative stage.
As you settle, the fast beta chatter fades and slower, taller waves take over. That handover is the story of falling asleep, written in real time.
Finding “sleep onset”
The moment beta gives way to steady alpha and the first theta appears is what scientists call sleep onset — the exact minute you cross from awake to asleep. Most people can’t name that minute; the brain doesn’t announce it. The mask can. By watching the balance of bands, it knows when you’ve tipped over and can ease its warmth and audio down so nothing pulls you back.
Why the forehead?
The frontal region sits close to the surface and stays clear of hair, so a comfortable dry contact gets a clean read without gel or clips. It’s the most practical place to listen all night without you ever noticing the sensor is there.
Measured, not guessed
Most consumer sleep trackers infer your night indirectly. A wrist device watches movement and heart rate and makes an educated guess about which stage you were in. It’s clever, but it’s a proxy — lie still while wide awake and a motion tracker will happily score it as sleep. EEG skips the guesswork. It listens to the organ that actually does the sleeping, so the stages it reports are the stages you were in, not a statistical estimate of them.
What you see in the app
You don’t need to interpret raw voltage. In the morning the app translates the night into plain language: how long until you fell asleep, how much time in light versus deep sleep, when you stirred, and how it all compares to your weekly average. Over a few weeks those numbers start to tell a story — the late coffee that cost you deep sleep, the early night that paid you back. Because the signal comes straight from the source, the trends you act on are real ones. That’s the whole point of putting a lab-grade sensor into something you’d actually wear to bed: better data, and a better night to show for it.