Blame smartphone alerts, constant connectivity and a deluge of media for our society’s sleep deprivation. But the root cause of why we get less sleep now than our ancestors did could come down to a much simpler reason: artificial light. New research comparing traditional hunter-gatherer living conditions to a more modern setting shows that access to artificial light and electricity has shortened the amount of sleep humans get each night. The research is the first study to document this relationship in the field.
“Everything we found feeds what we had predicted from laboratory or intervention studies, where researchers manipulate certain aspects of light exposure. But this is the first time we’ve seen this hold true in a natural setting,” said lead author Horacio de la Iglesia, a University of Washington biology professor.
The researchers compared two traditionally hunter-gatherer communities that have almost identical ethnic and socio-cultural backgrounds, but differ in one key aspect – access to electricity. They wanted to see if, all other factors aside, electricity would impact people’s sleep during an average week in both the summer and winter. They found this rare scenario in north-eastern Argentina, with two Toba/Qom indigenous communities living about 50km apart. The first has 24-hour free access to electricity and can turn on lights at any time, while the second has no electricity, relying only on natural light.
In their usual daily routines, the community with electricity slept about an hour less than their counterparts with no electricity. These shorter nights were mostly due to people who had the option to turn on lights and go to bed later, the researchers found. Both communities slept longer in the winter and for fewer hours in the summer.
Though this study took place from 2012 to 2013, the sleep-pattern differences observed between the communities can be seen as an example of how our ancestors likely adapted their sleep behaviours as livelihoods changed and electricity became available, de la Iglesia said.
“In a way, this study presents a proxy of what happened to humanity as we moved from hunting and gathering to agriculture and eventually to our industrialised society,” he said. “All the effects we found are probably an underestimation of what we would see in highly industrialised societies where our access to electricity has tremendously disrupted our sleep.”
The researchers visited each community for a week during the summer and winter, placing bracelets onto the wrist of each study participant to monitor activity. The devices can track slight changes in movement, so a still wrist for a longer time implies that a person is sleeping. Participants also kept sleep diaries during the study period, where they recorded what times they went to bed and woke up, as well as any naps throughout the day. This information mainly was used to corroborate data collected from the wristbands.
Even in sub-tropical Argentina, where the differences between summer and winter daylight hours vary about two and a half hours at most, study participants naturally slept longer in the winter. In a high-latitude place like Seattle, that daylight difference is close to eight hours between summer and winter. These findings suggest there’s a biological driver in humans that requires more sleep in the darker winter months.
“We tend to think we’re isolated from seasonal effects even though we know this is the case for many animals,” de la Iglesia said. “I think it’s still embedded in our biology even when we do as much as we can to obscure that difference between summer and winter.”
Past research has shown that artificial light can disrupt our circadian clock and sleep-wake cycle, effectively pushing them back when we turn on the lights in the evening. The researchers have documented this from their observation study, and they plan to look next at whether the later sleep onset and reduced sleep in the community with electricity is due to a shift in the biological clock by measuring melatonin levels in the two communities.
Access to electric light might have shifted the ancestral timing and duration of human sleep. To test this hypothesis, we studied two communities of the historically hunter-gatherer indigenous Toba/Qom in the Argentinean Chaco. These communities share the same ethnic and sociocultural background, but one has free access to electricity while the other relies exclusively on natural light. We fitted participants in each community with wrist activity data loggers to assess their sleep-wake cycles during one week in the summer and one week in the winter. During the summer, participants with access to electricity had a tendency to a shorter daily sleep bout (43 ± 21 min) than those living under natural light conditions. This difference was due to a later daily bedtime and sleep onset in the community with electricity, but a similar sleep offset and rise time in both communities. In the winter, participants without access to electricity slept longer (56 ± 17 min) than those with access to electricity, and this was also related to earlier bedtimes and sleep onsets than participants in the community with electricity. In both communities, daily sleep duration was longer during the winter than during the summer. Our field study supports the notion that access to inexpensive sources of artificial light and the ability to create artificially lit environments must have been key factors in reducing sleep in industrialised human societies.
sleep timing South American indigenous communities natural light-dark cycle artificial light-dark cycle
The daily timing and amount of sleep has changed throughout human history, particularly as our species transitioned from nomadic hunter-gatherer groups to agricultural societies and, more recently, to industrialised ones. Whereas historical records depict that the onset of sleep at night coincided with the inexorable arrival of dusk, modern society has crafted a sleep schedule that is heavily influenced by protected, artificially lit environments. Whether natural or artificial, light is among the most important environmental factors regulating sleep. In humans, light induces alertness and entrains a master circadian clock that regulates the timing of sleep. Thus, the ability to control our own exposure to light through artificial means likely altered when we sleep and for how long. Although this is clearly supported by human studies in sleep laboratories, testing this hypothesis in the field has been complicated by the lack of appropriate human populations. Although it is not difficult to find communities without access to electric light, it is difficult to find communities of the same ethnic and sociocultural background with free access to electricity that can be studied simultaneously.
The Toba/Qom are one of the 3 main indigenous groups in the Argentinean Gran Chaco region (provinces of Formosa and Chaco) of northeastern Argentina. The Western Toba represent a subgroup living in the western region of the province of Formosa (23°47′ S, 61°48′ W). Traditionally hunter-gatherers, they now exhibit a spectrum of subsistence and lifestyle patterns. Some live in neighbourhoods on the outskirts of small towns, whereas others live in relatively isolated villages of 20 to 600 people and still rely on hunting and gathering for at least part of their subsistence. We studied 2 Western Toba communities within this region. The first one has 24-h access to electricity (Electricity); the second one lives ~50 km away and has no access to electricity (NO-electricity). This latter community relies on daylight for their daily activities. To examine the hypothesis that access to electric light affects the timing and amount of sleep, we fitted participants in each community with wrist data loggers that measure motor activity and light exposure with high-frequency sampling, allowing quantification of sleep duration and timing.