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the best kind of sleep

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Canto: So Dr Seheult tells us that the most important sleep is in the earliest period. This is called slow-wave sleep or N3 sleep. It doesn’t last long, maybe half an hour…

Jacinta: Why is it called N3 sleep?

Canto: Well, here’s the detail – you have three stages of this early sleep. N1 is when you fall asleep. It lasts no more than ten minutes generally until you’re really there, in sleep. Then there’s N2 of course, which lasts from 30-60 minutes, your muscles relax and you begin to enter this slow-wave, also called delta-wave or delta brain activity sleep. That’s the deepest sleep of the night, at its deepest in the N3 period.

Jacinta: Well that explains the 1-2-3, sort of, but what about the N?

Canto: I haven’t been able to find that out specifically, but these are all phases of NREM (non-rapid eye movement) sleep, which are followed by the REM phase. So I think the N is just short for NREM. Anyway there are two types of sleep for maintaining good health – slow wave sleep and REM sleep towards the end of the night…

Jacinta: But don’t those two make up the whole of the sleep cycle?

Canto: Let me tell the story. Slow wave sleep is when you secrete valuable growth hormone, vital for children, in the time before midnight, according to the good doctor.

Jacinta: But I virtually never sleep before midnight.

Canto: Well you’re not alone there. In cities now, which are growing ever larger, we’re going to bed later and getting up earlier, and so sleeping less…

Jacinta: But generally living longer. So what’s the problem? I’ve heard that Hong Kong, which is about as urban as it gets, has the longest life expectancy on Earth – but that was probably measured before the China crackdown haha.

Canto: Well it’s no joke that China’s thugocracy will jeopardise everything in HK’s future, but good public healthcare and a very low infant mortality rate helps. People today can still live well with diabetes, obesity and slow-developing cancers, but they’d be even better with good sleep habits, if the rat-race allows them. But cities present us with a kind of eternal daylight, at great cost, not only in electric lighting, but in lack of sleep. Not to mention brightly lit screens that we take to bed with us…

Jacinta: Okay so what are the other benefits of slow wave and REM sleep, however delayed?

Canto: Dr Seheult describes a study showing that general sleep deprivation actually reduces the levels of antibodies produced after influenza vaccination. That’s to say, vaccination is less effective for the sleep-deprived. Another study used rhinovirus, a common cold virus. They paid students to be infected and found that those with good sleep efficiency, that’s to say, a high ratio of in-bed time to sleep time – their risk of being infected was reduced five to seven-fold, an extraordinary result. Actually this ‘extraordinary result’ finding comes up again and again in Matthew Walker’s book.

Jacinta: Yes, but it’s surely good to be awake sometimes too. But again, what is it about slow-wave and REM sleep that provides such benefits. What are the mechanisms?

Canto: Well, we’re talking about N3 sleep, the deepest sleep. This sleep phase is particularly important for memory consolidation, the stabilisation of a memory trace once it’s been acquired – meaning presumably the event itself, or its impact. It’s also called sleep-dependent memory processing. Now, how this precisely works is still being researched, but it appears to have much to do with interactions between neurons or neuronal complexes in the neocortex and the hippocampus. So here I should introduce sleep spindles, which are essential to all mammalian species.

Jacinta: They’re brainwaves, aren’t they?

Canto: Neural oscillations, indeed. They’re generated in the thalamic reticular nucleus (TRN), in richest quantities during N2 and N3 sleep. Wikipedia tells me this:

The density of spindles has been shown to increase after extensive learning of declarative memory tasks and the degree of increase in stage 2 spindle activity correlates with memory performance.

This is confirmed in experiments described in Why we sleep, showing that people who slept for a night between being asked to memorise certain data, like putting a name to a face, did a significantly better job than those who tried to remember the data after eight hours without sleep (from morning to evening). During the sleep period, subjects’ brain waves were recorded, and this is Dr Walker’s account:

The memory refreshment was related to lighter, stage 2 NREM sleep, and specifically the short, powerful bursts of electrical activity called sleep spindles… The more sleep spindles an individual obtained during the nap, the greater the restoration of their learning when they woke up. Importantly, sleep spindles did not predict someone’s innate learning aptitude. That would be a less interesting result, as it would imply that inherent learning ability and spindles simply go hand in hand. Instead, it was specifically the change in learning from before relative to after sleep, which is to say the replenishment of learning ability, that spindles predicted.

Jacinta: So they were correlating the number of spindles with their memorising performance, and memory here is being equated with learning. Is that right? I mean, is learning really just memorising?

Canto: Well, no, but it helps. I’m trying to memorise Newton’s inverse square law for gravity, but I know that even if I can reel it off like a favourite poem that doesn’t mean I fully understand it. Let me see G = m1.m2 over r². I’m not sure if that’s right.

Jacinta: Yeah, basically you have to know that the gravitational attraction between two bodies is equal to the product of their masses divided by the square of the distance between their ‘centres of mass’. I think. Though why that happens to be the case I have no idea. Does anyone?

Canto: Because… the universe? I’m beginning to feel sleepy…

References

M. Walker, Why we sleep, 2017

https://www.uofmhealth.org/health-library/hw48331

How to get the best sleep for your immune system | Roger Seheult (video)

https://www.oal.cuhk.edu.hk/cuhkenews_202101_life_expectancy/

https://www.ncbi.nlm.nih.gov/books/NBK526132/

https://en.wikipedia.org/wiki/Sleep_spindle

Written by stewart henderson

November 14, 2021 at 12:54 am

Posted in memory, mind, science, sleep

Tagged with , , ,