Matt: But also I would say REM sleep, too, you know, I think REM sleep takes a backseat. At sort of, you know, a bit of a neglected stepsister in the sort of sleep conversation. REM sleep, we found all manner of different functions, one of which is emotional first-aid. It's incredible for palliative emotional benefit. It's not time that heals all wounds, but it's time during REM sleep that provides emotional convalescence.

Rhonda: I guess that would also then be important to help you sleep better, because if you are managing your emotions better, you probably have less anxiety...

Matt: Correct. Exactly. But also REM is for the body, too, you know. It seems to regulate cardiovascular function. So REM sleep is critical too.

Rhonda: Oh, really? Is REM what's important for lowering the blood pressure?

Matt: No. Well, it's during deep sleep that you get this lowering of blood pressure, and we've got some data right now that we're about to publish that deep sleep provides actually a homeostatic recalibration of blood pressure.

What do I mean by that? If I measure your blood pressure before sleep and then after sleep at a matched circadian time, so we remove the circadian fluctuation of which there is a large one to your systolic blood pressure, but we match it, and so the only thing that's different is the quality of your sleep, what we've discovered firstly is that the amount of deep sleep that you have measured in these big delta slow waves during deep non-REM sleep, that predicts how much of a drop in your systolic blood pressure you will have in the morning relative to the evening as if deep sleep provides a recalibration of the cardiovascular system.

Rhonda: Wow.

Matt: If you're having higher frequency brainwaves that are not like the deep sleep, that's what we call unrestorative sleep, if you have a ratio of very little deep brainwave activity and a lot of high-frequency wake-like brain activity, what we call delta-beta ratio, that predicts very bad cardiovascular outcomes, that if anything, your systolic blood pressure is even worse after sleep than it is the night before.

And what we found is that during aging, now we looked at this and healthy people, then we replicated in older adults, and we found that the older that you get, the less deep sleep that you get, the more sort of wake-like or faster-frequency brainwave activity, which isn't good during deep sleep. And that accurately predicts the cardiovascular dysfunction that we see in aging. So we now think we understand in part why poor sleep is linked into poor cardiovascular outcomes.

One of the benefits of sleep is that it resets cardiovascular tone and particularly systolic tone. It doesn't stop there. There's lots of other benefits, heart rate drops, etc. But REM sleep also seems to have this strange function where you go through these cycles where you get a massive activation of the cardiovascular system, and then it falls silent, and then a massive escalation again. And you get this really remarkable increase in heart rate variability.

And heart rate variability has been used as a metric of health outcome, that if you have high heart rate variability, it's very good. It predicts lots of health outcomes. That's what you get during REM sleep. So we should be careful not to think of everything being all about deep sleep. It's about all stages. Every stage of sleep that we have, we have ascribed a function to. Which makes sense, you know, during sleep...Sleep is the most idiotic of all things in terms of a creation from Mother Nature. You know, you're not finding a mate. You're not reproducing. You're not eating. You're not caring for your young. And worse of all, you're vulnerable to predation. So on any one of those grounds, sleep should have been strongly selected against in the course of evolution.

The exact opposite is true. Every species that we've studied to date sleeps. What that means is that sleeper has fought its way through heroically every step along the evolutionary pathway, which must mean that if sleep doesn't serve an absolutely vital function, it is the biggest mistake that the evolutionary process has ever made. And if any one of those stages of sleep could be removed, because it was not important, because I want to, you know, emphasize this type of sleep because that other sleep must not be important, I promise you, mother nature would have thought, I'm gonna excise that stage of sleep out because you're just too vulnerable.

But I think, in your position, of course I would be concerned about my deep sleep, thinking about things like APOE-4, Alzheimer's risk, too. But I think the bottom line is that no matter which way you slice the sleep pie, you just can't shortchange sleep.