Members of the Himba community in Namibia get just 5.5 hours of sleep a night on average
Nick Fox / Alamy
Much has been written about how modern lifestyles mean we are no longer getting enough sleep, unlike our ancestors who lived in less technologically advanced times. But an analysis of 54 sleep studies conducted around the world has found that people in small, non-industrialised societies actually get less sleep than those in more industrialised regions.
“Everyone I talk to in Canada and the US talks about how awful their sleep is,” says Leela McKinnon at the University of Toronto Mississauga in Canada. “The numbers aren’t showing that.”
It is often assumed that the rise of gadgets like big-screen TVs and smartphones mean that people today are sleeping less than in the recent past – the so-called sleep-loss epidemic.
But many studies that report a decrease in sleep in the past few decades are based on asking people how long they sleep, which is an unreliable measure. Even using this method, the results are mixed, with many studies finding no change or even an increase in sleep duration.
Research based on more reliable measures, such as physical activity monitors or using electrodes to monitor brainwaves, hasn’t found a decrease over recent decades. For instance, a 2016 review of 168 studies found no decline in sleep duration over the past 50 years.
But these studies were done in industrialised countries, leaving open the question of whether people got a lot more sleep prior to industrialisation. With the availability of wrist-based activity monitors, it has become easier to study sleep in non-industrialised societies.
Such studies have revealed surprising short sleep durations. For instance, among hunter-gatherers, the San sleep for 6.7 hours a night on average, the Hadza for 6.2 hours and the Bayaka for 5.9 hours. The shortest duration found so far is the 5.5-hour sleep of the Himba community in Namibia, who are nomadic livestock herders.
McKinnon and her colleague David Samson, also at the University of Toronto Mississauga, have been involved in several such studies. They have now compared sleeping habits in industrialised societies, including the US, Australia and Sri Lanka, with those in smaller, non-industrialised communities, including Indigenous peoples in the Amazon, Madagascar and Tanna Island in the Pacific.
Altogether, the analysis is based on 54 studies that involved direct measurements of sleep in people aged over 18 who had no serious health conditions. While these studies involve only 866 people in total, the dataset is the most comprehensive to date, says Samson. “It’s the best there is right now.”
Overall, these individuals slept for 6.8 hours on average, but in non-industrialised societies, the average was 6.4 hours, compared with 7.1 hours in industrial societies.
The pair also found that people in non-industrialised societies were asleep for 74 per cent of the time they were in bed, compared with 88 per cent in industrial societies, a measure known as sleep efficiency.
McKinnon and Samson also assessed the regularity of people’s circadian rhythms using a measure called the circadian function index, where a score of 1 is perfect. In non-industrialised communities, the average was 0.7, compared with 0.63 in industrial societies.
Samson attributes the higher sleep duration and greater sleep efficiency in industrialised societies to conditions more conducive to sleep. “We see that we’ve made some real gains in the safety and security of our sleep sites,” he says. “We don’t have to fend with rival human groups at night or predators.”
On the flip side, people in industrial regions are less exposed to the cues that help maintain circadian rhythms, such as lower temperatures at night and bright light exposure during the day. While they didn’t assess this, McKinnon and Samson both suspect that having less regular circadian rhythms could have adverse effects that explain why many people perceive their sleep to be poor.
What isn’t clear from the paper is how representative the individuals in these 54 studies are of their overall populations, says Nathaniel Marshall at Macquarie University in Sydney, Australia. “In order to make statements about prevalence in epidemiology, you need to have representative sampling,” he says.
Samson says he did look at whether having larger sample sizes could change the results, and they concluded that it wouldn’t make a large difference.
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