Just so? How the late sleepers got their Zs
Evolutionary biology doesn’t have an explanation for your struggle to start the day early — but we shouldn’t need that explanation in the first place.
I’m not much for New Year’s resolutions, but here at the beginning of 2021 I’m taking another swing at a habit I’ve tried to build up for most of my adult life: waking up early, naturally. I’d love to be a morning person, rising easily to finish breakfast and a workout before starting my workday. I can start myself on that schedule, especially after a restful period like the winter holidays, but keeping it up is the challenge. One night I’ll stay up past 10 reading or putting in extra time on a project — then the next day starts late and runs correspondingly later, and then the next day after that, and then the next. Eventually I’ve ratcheted myself to rolling out of bed at the last possible moment, and I’ll need more than a weekend’s worth of reset to go the other way.
I might be fighting my own physiology, here. A recently rebroadcast episode of NPR’s Short Wave podcast interviews Syed Moin Hassan, a medical doctor and sleep specialist, who has strong objections to the idea that waking up late is lazy, or even undesirable: people simply differ in their daily circadian rhythms, which are determined by fundamental biology. Societal associations between early rising and industriousness notwithstanding, it’s quite possible to be healthy, well-adjusted and productive without rising at the crack of dawn. This is, really, a reminder I need to hear.
But what surprised me was that Hassan also suggests there might be an adaptive reason that different people have different sleep patterns:
Evolutionarily … society needed people with differing “clocks.” For example, the late circadian sleepers, people who sleep late and wake up late, were found to be beneficial because they would protect the tribe, right? They would be up, they would protect them from predators, and things at night.
As soon as I heard that, I wasn’t listening as a regular Public Radio audience member — I was listening as an evolutionary biologist. Hassan was describing a specific process of evolution, and I had to know whether it could actually be true, or whether it was only a just-so story.
Just so? So what?
Evolutionary biologists generally do not approve of “just-so stories”. The phrase comes from a collection of children’s stories by Rudyard Kipling, each of which explains a feature of biodiversity. “How the Camel Got His Hump” describes a djinn punishing a lazy camel by giving it a hump to store water and food so it can be made to work longer between breaks; “How the Leopard Got His Spots” has the titular feline asking a human friend to paint his fur for camouflage. They’re fanciful, but they are, in a sense, doing what evolutionary biology aims to do: explaining how the world around us came to be. In that sense, evolutionary biologists are trying to write true just-so stories. However, we more often use the phrase as a taunt — calling an evolutionary explanation a “just-so story” usually means that it’s as fact-free as one of Kipling’s tales.
An evolutionary just-so story usually diverges from Kipling, however, by citing natural selection. If a trait or behavior increases the number of offspring an individual carrying the trait will have, and if parents carrying the trait pass it on to their offspring, there should be more individuals carrying the trait in the next generation. That’s natural selection, as first outlined by Charles Darwin and Alfred Russel Wallace, and it’s held up as evolutionary biology became the backbone of modern biology over the last century and a half. Evolutionary just-so stories generally describe how a trait or behavior might be favored by selection — how they increase the odds of survival or the number of offspring an individual might have — but that description is really just a hypothesis if it’s not supported by specific kinds of evidence.
Sleeping on the savannah
To validate the evolutionary scenario Hassan describes, we need to know whether sleep habits have a genetic basis, and whether they make a difference in survival or reproduction that could drive natural selection. Research in model species like fruit flies and mice has indeed found specific genes that contribute to daily, circadian, behavioral rhythms, and variants in the human versions of those genes are associated with whether participants in the 23andMe and UK BioBank research cohorts describe themselves as “morning people”. There is also, surprisingly, a little evidence linking sleep habits to reproductive success, from a recent survey of students and employees at a university in Germany. However, it’s in the opposite direction we’d need for natural selection to favor late risers: survey participants who described themselves as evening people tended to have fewer children.
Then again, Hassan isn’t really talking about natural selection based on an individual advantage. He says late sleepers would stay up at night and “protect the tribe”. That’s not really conventional natural selection — it’s what we call group selection. The idea is that, if a trait provides a benefit to a whole community group — like protection from predators late at night — and that benefit gives the group an edge against competing groups, the trait might persist even if the individual people who carry it have fewer children. Group selection is not as widely and uniformly accepted as natural selection, though there are conditions in which we do think it can operate.
I think I’ve found the evidence supporting this group selection scenario, too: a 2017 study of daily activity in a community of hunter-gatherers in Tanzania. The authors of that study asked 33 volunteers in a Hadza community to wear “actigraphs” — simplified fitness trackers, basically — for a period of 20 days. Data from the trackers revealed that, on average, 40% of the volunteers were awake at some point over the course of a night; and, again on average, more than one volunteer was active at any given time of day.
The authors analogize this to “sentinel” patterns of daily activity in other social animals, and conclude that it provides a similar benefit to the community of low-level vigilance for predators (or human threats). They also specifically suggest that group selection might favor a variety of sleep patterns within small communities like the one they tracked. If we accept that the modern Hadza lifestyle parallels that of early humans, this at least demonstrates that there’s a plausible benefit to living in a community where not everyone sleeps on the same schedule.
It ain’t necessarily so
Plausible isn’t the same as proven, though. It takes more than a few logical stretches to link the Hadza volunteers’ sleep habits to my personal difficulty with early rising. Notably, knowing there’s a genetic basis to sleep habits in research cohorts like the UK BioBank doesn’t mean the variation among those Hadza volunteers is due to their genes. In small communities, it’s not uncommon for everyone to be more closely related than a random sample of the broader population. The authors of the 2017 study didn’t check for this, but if it’s the case, it could mean that the variation in the volunteers’ circadian rhythms is present in spite of their having similar genetic inclinations to late or early rising.
Indeed, another pattern in the activity tracking data was that older volunteers tended to rise earlier. Traits that change over the course of a person’s life can have a genetic basis, and evolve via natural selection. However, this means that the variation in sleep patterns that ensures someone is awake and alert at most times of day in this study is not really the same as what’s been examined in the BioBank and 23andMe cohorts.
Finally, there’s no direct evidence that the Hadza volunteers’ collective pattern of activity actually does improve defense against predators — for that you’d need to track predator risk, and you’d need data from a group of people who all slept on the same schedule as a control. Even if you could collect that data and found a measurable difference, there would be a further question of whether the difference was big enough to give a community of hunter-gathers a meaningful advantage over other similar groups, so that group selection could operate.
All that information would be challenging to collect, particularly with human subjects. Even in non-human species, piecing together the data to take an evolutionary scenario from “plausible” to “proven” can be the work of a whole scientific career. But that doesn’t mean we shouldn’t take the difference seriously — and that’s why I have to conclude that Hassan’s original citation is, in the end, only a just-so story.
What’s most vexing about this particular just-so story, though, is that I’m entirely sympathetic to the larger point Hassan is making — that people naturally differ in their sleep patterns, and that society ought to be able to accommodate that diversity. As he also says in that interview:
But now what’s happening is we are trying to fit everyone into the same box. And … like height, like intelligence, like hair color, your circadian clock varies across the population. So we’re not appreciating the individual uniqueness of every person, and that’s where our problem is coming.
Especially after spending most of the last year learning how to conduct business online and asynchronously, it’s plain that human ingenuity gives us options to accommodate those of us who struggle out of bed in the morning and find renewed focus after dinnertime. Invoking our evolutionary history might seem to strengthen the case for those accommodations, but it shouldn’t be necessary — and, as we’ve seen, it doesn’t seem to be supported by the available evidence. Human beings aren’t all identical, and we have it in our power to treat our differences more kindly. We don’t need permission from natural selection to do that.