Rhonda: I remember reading somewhere that when you sleep, and this is related to what you were just talking about, that your brain sort of replays, like, electrical-activity-wise, it looks like you're literally, like, reliving the same thing you just learned or something. Is that...

Matt: That's absolutely correct. So these studies firstly happened in animals, and we've now been replicating some of them with brain imaging, with MRI scans in humans. Hard to believe, but we can do it. But the original findings were fascinating. They would place electrodes into that structure that we spoke about, the hippocampus, which also rats have as well, and it helps rats learn spatial navigation.

And they would place lots of electrodes into this part of the brain. And as the rat would run around the maze and learn the maze, individual cells would fire, and they would spatially code which part of the maze the rat was in. So different cells are mapping different parts. It's like sort of navigating a route from your home to work. Different sort of cells are coding the journey along the way.

Rhonda: Yeah. I'd probably would suck at doing that, but...

Matt: I think, yeah, yeah. I lost that. Yeah, some people say it's a male aspect of the gene, but I definitely lost it. Anyway, but what is delightful was that you could sort of hang, you know, a sound tone on each one of these electrodes. And what you would hear, and this is just, you know, for audio, you would hear sort of "buh buh buh bump, buh buh buh bump, buh buh buh bump," as the rat was running around the maze, as these cells were learning, encoding, and creating this kind of memory circuit essentially.

And, yeah, you know, "buh buh buh bump," off it goes. But what was genius is that they kept recording. And as the rat fell asleep, what did they hear? And it wasn't just static random firing, which is what we thought typically happens during sleep. In that static of electrical impulses at night, in a sea of electrical noise, came out a very clear, predictive, message, which was "brrrmp, brrrmp, brrrmp." It was exactly the same temporal sequence "buh buh buh bump, buh buh buh bump, buh buh buh bump," but it was sped up. And we now know that it's during sleep that we replay, but we replay at somewhere between 10 to 20 times the speed.

So it's as though you're kind of, you know, you've done the, you know, recording of whatever happened during the day, but then it gets replayed but at times 20 or times 10. "Brrrp, brrrp, brrrp, brrrp, brrrp."

Rhonda: Wow. So is this is a long-term potentiation part? Is this where it's solidifying?

Matt: Yeah.

Rhonda: Now what stage of sleep is that? I know you mentioned...

Matt: So that's during deep non-REM sleep.

Rhonda: Deep.

Matt: Yup, that we see that.

Rhonda: It's the sleep I'm always trying to optimize for.