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In a new study, anthropologists at the University of Toronto and the University of Nevada monitored sleep in a modern-day hunter-gatherer tribe and found that, no matter the time of night, some portion of the tribe was always awake.

The study, published in Proceedings of the Royal Society B, marks the first time that researchers have tested out the sentinel hypothesis in humans. The gist of the hypothesis is that, in dangerous situations, a group of people (or animals) only goes to sleep if sentinels (watchdogs) take turns staying vigilant. The hypothesis explains variations in chronotype (larks and owls) as a mechanism that evolved to allow early humans, who slept in groups, to fall into a deep, defenseless sleep without making themselves too vulnerable to environmental threats. 

The psychologist Frederick Snyder floated the sentinel hypothesis back in 1966 to explain group-sleeping behavior in humans and other species. There are plenty of situations in which people are required to stay up for part of the night so that the rest of their group can safely grab shuteye. In the military, for instance, troops take turns with nighttime guard duty. But the sentinel hypothesis doesn't suggest that a group of sleepers actively appoints sentinels. Rather, the idea is that natural variations in sleep-wake timing, as well as the occurrence of nighttime awakenings and periods of light sleep, function as adaptations to "increase group-level vigilance and survivability as a way to counter outside threats." Someone is always alert enough to detect danger. And "alert enough" could either mean being fully awake or being in a light, easily disturbed state of sleep.

In the current study, researchers wanted to investigate the sentinel hypothesis in humans. Since pills, screens and alarm clocks have changed how and when the Western world dozes, researchers sought out people whose sleep behavior remains untouched by technology and other modern-day conventions, such as the 9-5 workday. They zeroed in on the Hadza hunter-gatherers of Northern Tanzania, whose non-industrialized lifestyle is as close to that of ancestral humans as it gets in 2017: no artificial light, storebought food or EOD deadlines.

For nearly the whole time it was dark outside, at least some people were always awake.

Researchers predicted that "only rarely will all individuals in a group be identified as asleep during night-time periods." To conduct the study, researchers outfitted 33 Hadza participants with actigraphy watches (e.g., fitbits). Actigraphy isn't the most reliable method of collecting sleep data because it assesses sleep and wakefulness based on movement. As a result, someone who is lying awake, but motionless, might register as asleep, while a fitful dreamer might be deemed awake. Even so, misinterpreted wakefulness would most likely reflect a light sleep stage marked by partial responsiveness to sounds, sights and smells in the environment. 

At any given minute-long interval during the night, researchers found, about 60 percent of the Hadza were asleep while 40 percent were either awake or close enough. Researchers also identified a 12-hour-long "group sleep time," which is the amount of time between the first person falling asleep at night and the last person waking up in the morning. The group slept for about twice as long as individual members did. This means that, for nearly the whole time it was dark outside, some people were always awake.

Chronotype varied across the Hadza, but only one variable predicted sleep timing: age, specifically old age. Based on this finding, researchers came up with the "poorly sleeping grandparent hypothesis" as an alternative explanation for the sleep changes that people undergo later in life. It's possible, they argue, that we've jumped to the conclusion that older people who find themselves sleeping less, and on an earlier schedule, have sleep disorders requiring treatment. Maybe they're just "wakeful grandparents" carrying out their evolutionary mandate to work the sentinel shift. 

Maybe "wakeful grandparents" are just carrying out their evolutionary mandate to work the night shift.

Overall, researchers found that variations in chronotype and periodic awakenings spared the Hadza from needing to implement any formal sentinel system. Animal behavior studies have widely reported an inverse relationship between group size and vigilance. And research has shown that smaller groups of hunger-gatherers actively rotate sentinels. These insights, taken together, suggested to researchers that natural sentinel-like behavior may only occur in groups of a certain size.  

Based on the findings, researchers wrote, it would be worth exploring the possibility of an optimal group size and age-mix for naturally occurring sentinel behavior: If a group is too homogenous in age, or too small, then chronotype might not vary enough to support sentinels all night long. And, in small groups, people might be too anxious about surviving through the night to let themselves fall into a non-vigilant sleep state. This is consistent with the "First Night Effect," a protective mechanism wherein only half of your brain fully powers down on the first night of vacation. Once your brain accepts that your Ramada Inn suite is a safe space, then you're able to enjoy whole-brain shuteye.

It's easy to build on this study because it's relatively original. In the future, researchers could use more sophisticated tools and methods, such as a combination of EEG analysis and artificially controlled levels of threat, to take a more in-depth look at the relationship between threat detection and sentinel-like behavior. But, it's worth noting, evolutionary theories can be controversial. So, expect this study to generate a lively debate (in the sleep world, at least).