Dr. Matthew Walker

What they're referring to is dreaming that takes place during the stage of sleep called rapid eye movement sleep or REM sleep.

And during REM sleep, there we have these bizarre hallucinogenic, these vivid, these narrative, these emotion-filled story experiences.

Those are the types of reports that we get principally from REM sleep.

So if I were to go back to episode one and say, think about those different stages of sleep, light non-REM sleep, deep non-REM sleep and REM sleep.

When does dreaming occur?

Well, if I wake you up during stage two non-REM sleep, that's one of the lighter forms of non-REM, early in the night, but especially later in the night, you typically will report a dream, maybe about 50% probability.

50% of the time I wake you up out of that stage, no report.

The other half, yes.

If I wake you up to a deep non-REM sleep, stages three and four, we're down to a zero to 20% chance that you'll report a dream.

So very, very unlikely.

If I wake you up out of REM sleep, somewhere between 80 to 90% probability that you were going to report a dream.

There's nuance in that REM sleep story, by the way.

Rapid eye movement sleep is defined by those rapid eye movements.

But when you're in that stage of sleep, you're not always having the eye movements.

They come in these strange phases.

So you will be in REM sleep, and we can define that with lots of different sensors on your brain and your body.

But then there will be times when your eyes are darting back and forth and times when your eyes are not.

And when those eyes are moving during REM sleep, we call that phasic REM sleep.

And when they're not, we call it tonic REM sleep.

Don't worry about the terminology.