Dr. Matt Walker

So as we start to fall into those lighter stages of sleep, once you get past stage one sleep, which is sort of almost the shallows where you're just wading out,

Then you go into stage two sleep.

And one of the hallmarks of stage two non-REM sleep are something called sleep spindles.

And the way that we measure sleep in a laboratory, by the way, is that we place, you look like a spaghetti monster.

You've got all of these electrodes on your head.

You've got things above your eyes and you've got things on your body.

And we're essentially measuring three main signals, electrical brain activity, we're measuring muscle activity, and we're measuring eye movement activity.

And I'll explain why those three things are necessary for me to know, are you awake?

Are you in sleep?

And if you're in sleep, which stage of sleep you're in.

So going into that stage two non-REM sleep, we've got these sleep spindles.

And at that point, I'm looking at the electrical signals from your brain, what we call the EEG or the electroencephalogram.

And these sleep spindles are these beautiful, short, synchronous bursts of electrical activity.

And they last for about a second to two seconds, maybe a little longer.

And they are bursting at what we call a frequency of somewhere between 12 to 15 Hertz.

And what that means is that these brain waves are going up and down 12 to 15 times per second.

That's what our measure is, 12 to 15 Hertz.

And then you go back and your brain at that point has started to slow down.

Now, when we're awake, your brainwave activity can be going up and down, maybe 20, 30, 40 times per second.

It's very fast and frenetic.