Matt: So I mentioned that just coming back to the quality of sleep, when you position the quantity of sleep on the clock face, on the 24-hour clock face, at an inappropriate time according to your natural innate preference, and I say that specifically rather than a particular time, you will not get the same quality of your sleep. So an extreme version of this is a night shift worker, where they may get eight hours of sleep in bed but they are sleeping during the day, not during the night. So it's a complete reversal. The quality of their sleep is not good. They don't get the same amount of deep sleep nor do they get the same amount of REM sleep.

But let's kind of walk that back a little bit to your scenario, which is much more subtle. Let's say that my chronotype, and your chronotype simply determines whether you're a morning person, an evening person, or somewhere in the middle, about 30% of the population is an extreme morning type or a morning type, about sort of 40% percent is sort of neither strongly morning or evening, and about the remaining 30% is an evening type, we call them owls and larks.

So if you're a morning type, you like to go to bed, let's say, like, 9 p.m. And wake up at 5 or 6. If you're an evening type, you may want to go to bed at 1 a.m. And wake up at 9 a.m. The next morning. So when I'm saying the optimal position of sleep, I'm saying with the optimal for your chronotype, this is your chronotype...And by the way, it's genetic. You don't get to decide whether you're a morning type or an evening type. It's hardwired into your genes. We know the genes. It's not your fault. If you're an evening type and you're listening to this, and society, which is strongly architected against your chronotype, we reward and we favor the morning types, it is not your fault. It's not a choice. And there's not too much you can do about it. You can push and pull the system by about 30 minutes, 45 minutes, but not much more.

So coming back to it, when we position that sleep on that eight-hour period, whatever is optimal for you, you will get a nice distribution. You'll get all of the deep sleep that you want and all of the REM sleep that you want. And earlier in the conversation, I said that we go through these 90-minute cycles, we, human beings, it's different for different animals, you always go into deep non-REM sleep first and then you always have REM sleep second. And that repeats every 90 minutes throughout the night.

What changes, however, is the ratio of non-REM to REM within those 90-minute cycles as you move across the night such that in the first half of the night, the majority of those 90-minute cycles are comprised of lots of deep non-REM sleep and very little REM sleep. In the second half of the night, that balance shifts, and now you get much more REM sleep in the late morning hours and very little deep sleep.

The reason I'm saying this is because let's take someone who goes to bed at a substandard amount of time. Let's say that they go to bed at midnight, and they're going to wake up at 8. But instead they have to wake up, because they've got an early morning meeting, so they go to bed at midnight and they wake up at 6. And my question is how much sleep have they lost? And your response is, well, they've lost two hours out of the eight hours, which means they've lost 25% of their sleep. And your answer would be right in a way, but it would be wrong as well, which is that they've lost 25% of their total sleep but they may have lost almost 80% of their REM sleep because that's the REM rich phase of the night.

And it also works the other way around, too, that if you go to bed too late, you will...So what happens is, because of the circadian rhythm, the brain has a different appetite for different stages of sleep on the 24-hour clock face. In the late evening and the early morning hours, the brain has a dietary preference for deep sleep. And it doesn't very much have an appetite taste for REM sleep. Through to the second half of the night, that's when it gets its appetite for REM sleep.

So if you start sleeping at 4:00 in the morning and you wake up it in the middle of the day, your brain has lost its appetite for deep sleep. And so it won't get much. Now you will probably get much more REM sleep as a consequence. So you've got to be really careful. That why...

Rhonda: So it's almost like your chronotype may even dictate in a way how much deep sleep you may get.

Matt: The chronotype will...If you sleep naturally and you get the amount of sleep that you want, your chronotype will probably mean that...your circadian rhythm is actually shifted as your chronotype, too. So you will still get, as an owl, if you get your eight hours when you want it, you will probably get a similar sleep architecture as a lark, who has gone to bed five or six hours earlier, because your circadian rhythms are five to six hours different.

But you're right in the sense that owls typically try to go to bed early. But because they are designed not to fall asleep at that time, they're just gonna lie in bed. Many owls think that they have insomnia. They don't. They're just not going to bed at the right time. Because they get into bed, and it's like a teenager, whose rhythm is also shifted late, you tell them to go to bed because you've got to wake up for early school start times, but there's nothing they can do because their circadian rhythm has shifted forward in time. And it's the same for owls.

So what will happen is that they will probably stay awake for a while, then they'll get into deep sleep. But then they have to wake up at an earlier time, and they will lose a lot of REM sleep. REM sleep, for emotional well-being, what the owls typically experience: depression, low mood, anxiety. So I think we're really starting to put the pieces together on that component.

Rhonda: And so for someone like me, I'm somewhere in the middle, like, when I don't...pre-baby. Now I'm I guess you would call a lark because I'm going to bed at, like, 9. My optimal time, if I can go to bed when my son does, and it's like, you know, if I go to bed, like, at 8, then I get the longer duration. That way, if I'm a little more fragmented, then I, like, at least can make up for it somewhat. But I'm up at 6 a.m. Now if it were up to me, I'd like to sleep...I usually would wake up, like, 8 a.m.

Matt: Yes, 8 a.m.

Rhonda: Like, that's my natural time.

Matt: And the reason that you can probably go to sleep sort of, you know, 9 p.m. Or even 8 p.m. Now is because you're chronically sleep-deprived. And as a consequence, you've built up such as sleep debt that, you know, there is a lingering what we call sleep pressure in your system. But, yeah, I think fighting your chronotype, we found, it comes with deleterious health consequences. Increased risk for poor cardio-metabolic outcomes. Things like, you know, C-reactive protein is higher. If you look at, you know, A1C in terms of sort of a Rorschach review of blood glucose, your blood sugar, not good. If you look at your propensity for being obese or being overweight, also not great if you're an owl and you're not sleeping according to your schedule.

Rhonda: Telomeres are shorter.