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Comment by rerx | Hacker News Reader

One of the many fascinating facts I learnt from Matthew Walker's "Why We Sleep" is that the biorhythms of teenagers are shifted to later hours compared to those of adults in all human cultures of the globe, even in those rare societies hardly affected by technology. In evolution this was likely beneficial to make it easier for teenagers to socialize among themselves without adult disturbance, which was necessary to drive their tribe out of its ruts, to pioneer innovation and adaptation to changed conditions. In modern society we are stupid to undermine this by depriving the upcoming generation of their sleep.

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This sounds like of a 'just so' story, as evolution is not usually considered to act at the group level, but instead at the individual level: https://en.wikipedia.org/wiki/Group_selection

And some evolutionary biologists, like Dawkins, argues that it really acts on genes rather than the individual.

So, I might be missing some of the claim, but I find it unlikely that teenagers would stay up later to be away from adults and drive the tribe out of its ruts.

Maybe so. But if it happens in all cultures across the globe (which is verifiable), then there must be ‘some’ cause. Even if we don’t know it.

That's true, but the core explanation is not likely to be what the original poster described.

You've misunderstood Dawkins.

You can still get completely unselfish individual traits out of selfish gene selection (like the entire chapter on runt gene selection). It looks like group selection, Dawkins only argues that the mechanism is selfish genes.

Even he seems to have forgotten he wrote about that though.

That's true, I just meant to express (over-simplistically) that he argued that the mechanism is at the gene-level, which is even farther away from group-selection mechanisms.

The sections about emergent group-level behavior are really fascinating too.

> easier for teenagers to socialize among themselves without adult disturbance

We've also disrupted this with younger kids. They never really get to play without adults in the mix.

I think this is actually one of the reasons why Minecraft was so successful, especially with kids. It gave them a way to explore the wilderness (also rare these days) and a way to interact without adult supervision. They got to be masters of their domain and learn how to cooperate and resolve disputes among themselves.

A better argument is presented by this excerpt from that book.

> humans likely evolved to co-sleep as families or even whole tribes, not alone or as couples. Appreciating this evolutionary context, the benefits of such genetically programmed variation in sleep/wake timing preferences can be understood. The night owls in the group would not be going to sleep until one or two a.m., and not waking until nine or ten a.m. The morning larks, on the other hand, would have retired for the night at nine p.m. and woken at five a.m. Consequently, the group as a whole is only collectively vulnerable (i.e., every person asleep) for just four rather than eight hours, despite everyone still getting the chance for eight hours of sleep. That’s potentially a 50 percent increase in survival fitness. Mother Nature would never pass on a biological trait—here, the useful variability in when individuals within a collective tribe go to sleep and wake up—that could enhance the survival safety and thus fitness of a species by this amount. And so she hasn’t.  

Thanks for giving me the inspiration to read it. This book is just fantastic.

I would argue that this ultimately has to do with one survival trait: who has more sex. I’m guessing that far more teen pregnancy occurs when teens are hanging out without adults around. The ability to shift to different sleep patterns than adults would be among other things, a procreational advantage.

You're mixing a modern model of family structure and sexual relations with evolutionary timescales.

It's very unlikely anyone gave a damn about who was having sex with who and at what age before our population sizes reached levels where paternity mattered.

Isn't who has sex with whom the primary issue in most animal groups?

Across the entire animal kingdom? I'm no expert, but that seems obviously to not be the case.

Edit:

Just look at our close relative the Bonobo for an interesting example of what human sexuality might have looked like in the past:

https://en.wikipedia.org/wiki/Bonobo#Sociosexual_behaviour

There actually have been studies of primitive tribes and cultures where it was showed that they just didn't realize pregnancy happens from a single man's sperm, so children were considered the entire tribe's or to have multiple fathers (with actual language terms for the various fathers). There's also some cultures where men first slept with men while growing up before settling down with women due to thinking sperm was a guy giving up his power and younger ones needing it to grow into warriors. So it wasn't really as clear cut as we know it to be today.

But evolution happens whether or not the creatures in question understand evolution or even heredity. (In fact, if they know nothing about it, that makes it easier for evolution to do its thing without interference.) The legendary "shape of the human penis designed to pull out the sperm of other men who have recently had sex with the same woman", if it worked, would do very well evolutionarily in the first kind of culture you describe. Likewise, if teenagers being awake at odd hours does cause them to have sex and reproduce more often, then (assuming the tribe will take care of the children) the genes that cause them to do this will be selected for, whether or not the teenagers or the adults realize it.

Exactly. If jealousy and mate-guarding behaviour evolved in some males, it might spread whether or not they understood what was going on or what the mechanisms are.

One could look at the animal kingdom to get more insight on this, though of course there is such huge variety there that it drives home the point that "it's complicated."

Perhaps the first question we should answer is if our closest primate relatives have this same behavior among their adolescence? Especially given to what extent adolescence is a social construct and not biological (which seems to clash with the idea this is found to happen in all societies, probably an interesting point to look more into).

Just as likely that teenagers would then be able to stand watch for predators, attacks by other tribes, or hunt for nocturnal prey.

Sure, having some variation in circadian rhythms among the members of a group makes it easier to have guards at most times of day and night. But such variation exists independently of age: There are "owls" and "larks" at all stages of life. I find it interesting that, on top of that, teenagers specifically and consistently have their individual rhythms shifted towards the later hours.

That's a really interesting point I hadn't considered. And thanks to evolution's pseudorandom algorithm, it could be that the benefit was obtained in conjunction with another benefit (nighttime socializing), that one came about after the other, or that one exists and one doesn't.

Can the purpose of this mechanism not be determined by observing hunter gatherers?

I don't think there's that many of those left, at least isolated and big enough to serve as an actual testing group.

> In evolution this was likely beneficial to make it easier for teenagers to socialize among themselves without adult disturbance,

Any evidence to put this down as a 'likelihood'?

Agreed, seems like a nice theory but without sources of any kind I could come up with another ten such plausible theories

I think it is more of an evolved response to predation. You need people awake to watch for threats. Teenagers could do that while adults tend to young kids etc... young kids and grandparents tend to awaken early etc... adults too wake earlier as they age so you reduce the threat window.

Maybe, but I wouldn't have put my teenage self on nightwatch. It wouldn't have been pretty. I'm also getting to the point where I don't think I would trust my father to be on early morning lookout.

Seems like there are enough adults who are night owls and morning larks to cover an adult-only watchfulness.

yea but we're not in tribes, are we? People take this evolutionary bullshit way too seriously, like we still run around with sticks and stones. None of that crap matters when we live in a modern society. Teenages are going to "socialize with themselves" whether they sleep in or not, no matter what, because, guess what, that's what people do in modern times. They can choose.

What they meant is that we evolved in a way that made teenagers having shifted cycles, going to sleep later and waking up later. We're not living in tribes anymore, but evolution is slow and we still cary these biological traits. It therefore hurts teenagers to wake up as early as adults. The point here is not about leaving them more time to socialize in the evening.

Wouldn't accommodating them make evolution slower? Those that have traits making them able to wake up earlier should be "rewarded" if you want to bring change.

Well, I didn't come to this thread expecting a eugenics-based argument in favor of medically unsound school start times, but here we are.

I was trying to comment on the irony in adapting to our evolution instead of the other way around. I wasn't trying to advocate any solution. Clearly it didn't come out right based on the replies I got.

Our goal is not to control evolution, it is to let people exist within the biological constraints they have.

Do you realize you're talking about killing people? Evolution happens when the less fit die or otherwise fail to reproduce. If you want to evolve more nocturnal teenagers, the least unpleasant implication is that you want late sleepers to stay sleep-deprived so they'll die in accidents.

We evolved to be remarkably adaptable species. Our social and environmental conditions influence so much, that there is zero reason to assume that any particular behavior in prehistoric time is fixed and not just adaptation.

Moreover, half of those "past evilutionary people were like this or that" is guess made out of flimsy evidence at best. It is best we have maybe, but not accurate. (For that matter, majority of popular "past people were x" is wrong even for periods where we have a lot of documentation of).

My anecdotal evidence is that when I do sleep at night, eat less and have a regular sex, my body works very well in my 30s. Things change instantly once you lower some grips. Something in me is not just adaptation, but finely tuned core mechanisms. In fact, our current live is very different from what serves well to us. And this is even exploited to large extents – see ads, politics, social irrationality. If it were mere adaptation, then we could be rational and adaptive af.

agree. also, it could just be a "evoulutionary side effect" related to something else and not for a reason.

Depriving people (especially, but not only, adolescents) off their sleep is disastrous for their mental health. For instance ADHD may be linked to lack of sleep (Walker suggests this in his book, I don't know how strongly this is backed by studies). Since by now we know that sleep times are driven by physiological factors to a large degree, I find it inhumane to not take these factors into account in scheduling.

Though it's not that simple; isolated use of sleep deprivation can be an effective treatment for at least one mental health issue: https://mosaicscience.com/story/staying-awake-surprisingly-e...

Walker sounds quite sceptical of this story in his book, sleep deprivation may be effective as treatment in some cases, but could be a cause of depression in more cases.

We can also eat nothing but Twinkies for our entire life. We live in modern society. We can choose.

As downvoted as this is, this has a point. People point too much to supposed "innate evolved tendencies" while completely discounting social construction and peer pressure and culture, which are definitely relevant.

Interesting idea :)

> was necessary to drive their tribe out of its ruts, to pioneer innovation and adaptation to changed conditions

Surely teenagers could scheme among themselves while adults were awake? All they'd have to do is walk somewhere else. It's not like the adults would constantly be in their way.

Also, those functions may as well be done by adults. For instance startup founders are not all kids right out of college.

Anyway, I'm not disagreeing with the idea that teenagers sleep later, just sceptical about why it is so.

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