Ever notice that some older people sleep less and sometimes more fitfully than younger people? Sure you did. On the other hand, you’ve probably also known old people who sleep in and some middle aged and younger people who complain of sleep problems. I could introduce you to representatives of all these groups from my own family.

Any phenomenon in human life is a suitable subject for evolutionary storytelling, and telling stories about the past is what a team of scientists set out to do. Traveling to Africa, they fitted members of the Hadza people of Tanzania with actigraph devices to monitor their sleep.

The subjects of the experiment are hunter-gatherers who slumber outdoors in small groups in the wild. The study confirmed what you would have guessed: It’s pretty typical for older people to doze and wake and doze and wake more than young people. They wake up earlier too. From Science Daily:

As part of the study, 33 healthy men and women aged 20 to 60 agreed to wear a small watch-like device on their wrists for 20 days, that recorded their nighttime movements from one minute to the next.

Hadza sleep patterns were rarely in sync, the researchers found. On average, the participants went to bed shortly after 10 p.m. and woke up around 7 a.m. But some tended to retire as early as 8:00 p.m. and wake up by 6 a.m., while others stayed up past 11 p.m. and snoozed until after 8 a.m.

In between, they roused from slumber several times during the night, tossing and turning or getting up to smoke, tend to a crying baby, or relieve themselves before nodding off again.

As a result, moments when everyone was out cold at once were rare. Out of more than 220 total hours of observation, the researchers were surprised to find only 18 minutes when all adults were sound asleep simultaneously. On average, more than a third of the group was alert, or dozing very lightly, at any given time.

They always have to put in that they were “surprised.” Otherwise why bother with the study? Anyway, that’s really all you need to know. Now take the challenge: With these facts, tell an imaginative story about human evolution. Think about it for a moment and if you want, write down your answer on some scratch paper.

Here’s a hint: Did you follow Monday’s news story from Colorado about a 19-year-old camp counselor, sleeping in the open by a lake, who awoke to find a bear sharpening its teeth on his skull? The counselor was camping out with four younger campers. He survived, despite serious lacerations.

OK, what did you come up with? If it’s the following, I wouldn’t be surprised: Long ago, ancient humans slept in the open, like these African hunter-gatherers. Having older people with you improves the likelihood that if hostile animals or people creep up, then someone will be awake to sound the alarm. Evolution selected for this feature. End of story.

We read on:

The findings may help explain why Hadza generally don’t post sentinels to keep watch throughout the night — they don’t need to, the researchers say. Their natural variation in sleep patterns, coupled with light or restless sleep in older adults, is enough to ensure that at least one person is on guard at all times.

…

[The scientists] call their theory the “poorly sleeping grandparent hypothesis.” The basic idea is that, for much of human history, living and sleeping in mixed-age groups of people with different sleep habits helped our ancestors keep a watchful eye and make it through the night.

“Any time you have a mixed-age group population, some go to bed early, some later,” Nunn said. “If you’re older you’re more of a morning lark. If you’re younger you’re more of a night owl.”

If the Colorado bear attack victim had been camping out with his grandparents, it’s possible one of them might have noticed the bear before it could get too close. Same thing if the group had posted a lookout, as I imagine some campers in the area, worried about bears, may do tonight. It’s genuinely surprising the Hadza don’t assign anyone to act as a sentinel. This would make me nervous if I were one of them, and it might even interfere with sleep.

In any event, is this science or storytelling? From the Abstract of the study:

Chronotype variation and human sleep architecture (including nocturnal awakenings) in modern populations may therefore represent a legacy of natural selection acting in the past to reduce the dangers of sleep.

It may or it may not. There are physiological reasons that sleep varies with age. Babies also wake up periodically during the night, while many teenagers have a hard time getting up in the morning. The story about ancient human lifestyles is interesting, but attributing “human sleep architecture” to natural selection is merely what chemist and National Academy of Sciences member Philip Skell, writing in The Scientist, dismissed as evolutionary “narrative gloss.”

Besides needing to say they are “surprised” by the results, the researchers, who published in their work in the Proceedings of the Royal Society B, are expected to come up with a reason why the findings are of practical usefulness. They oblige:

“A lot of older people go to doctors complaining that they wake up early and can’t get back to sleep,” [Duke University anthropologist Charlie] Nunn said.

“But maybe there’s nothing wrong with them. Maybe some of the medical issues we have today could be explained not as disorders, but as a relic of an evolutionary past in which they were beneficial,” said Nunn.

Some older people are bothered by their irregular sleep patterns. Others aren’t bothered by it. It’s the ones who are bothered that go to the doctor to complain. For them, it is hard to see what help a physician could offer, that would make a difference, by telling an evolutionary bedtime story.

Photos: Hadza people, awake and asleep thanks to evolution, via Duke University.