Scientists have long suspected that the discovery in the late 1800s of the electric light bulb — and the artificially lit environments it produces — has inexorably altered when and how long we sleep. The hypothesis has been supported by studies conducted in sleep laboratories, but no one has been able to test it in the field.
Until now. Last month, a team of researchers from the United States and Argentina reported on the results of a small but fascinating new sleep study involving traditional hunter-gatherer communities. The findings, published in the Journal of Biological Rhythms, appear to confirm the hypothesis that access to electricity and artificial light has significantly shortened the amount of sleep humans get each night.
For the study, the researchers recruited small groups of men and women (31 volunteers in all) from two Toba/Qom hunter-gatherer communities in the Gran Chaco region of northeastern Argentina. Both of these indigenous communities have the same ethnic and sociocultural backgrounds. They also live within 30 miles of each other.
But the communities are different in one major way: One has access to electricity and artificial light. The other does not. Thus, the two communities offer a natural setting in which to observe how the availability of electricity affects human sleep patterns.
All participants in the study were fitted with wrist devices that monitored their sleep-wake cycles for one week. They also kept sleep diaries to corroborate the data on the monitors. (The participants were literate enough in Spanish to fill out the diaries.)
The study was conducted twice: once close to the summer solstice in 2012 and again close to the winter solstice in 2013.
An analysis of the data revealed some rather dramatic findings. In the summer, the participants in the community with access to electricity slept, on average, about 40 minutes less per night than their peers in the community without access. Generally, the shorter sleep patterns were due to staying up later, for the participants in both communities tended to get out of bed at about the same time in the morning.
In winter, the difference in sleep duration was even greater: The participants in the community with access to electricity slept about an hour less, on average, than those in the community that depended on natural light.
Both groups slept longer in the winter than in the summer — a finding that suggests, say the researchers, “that a natural tendency to longer sleep as nights get longer is not fully overridden by access to electric light.”
Interestingly, the study found no evidence of an interval of “quiet wakefulness” in either community. Using diaries and other documents, some historians have argued that before the invention of the electric light, most people engaged in segmented sleep — their nights were divided into two distinct sleep periods with an hour or two of activity in between. (Another study, in which volunteers lived under Stone Age conditions, also found no evidence of sleep fragmentation.)
Limitations and implications
Like all studies, the current one has limitations. Most notably, it involved only a small number of people. Furthermore, although both of the traditional communities in the study live in similar types of housing and have similar social lives, more subtle differences in their daily habits — ones having nothing to do with access to electricity — might have influenced the results.
Still, the findings of the study, write its authors, are “consistent with the hypothesis that access to electricity has allowed humans to extend the evening light hours, thereby delaying the time of sleep onset and reducing daily sleep duration.”
Light affects human sleep in two ways, they explain: “First, light has direct effects on our ability to stay awake and active. This is in part, by enabling us to stay alert and engage in activities that are not possible in the dark but also by directly inhibiting physiological systems that facilitate sleep, such as the nocturnal onset in the release of the hormone melatonin. Second, light can directly reset the phase of the circadian pacemaker responsible for the timing of sleep, which is remarkably sensitive to environmental light.”
The bottom line: “Early humans may have had longer periods of rest than people in most industrialized societies experience today,” said Yale University biological anthropologist Eduardo Fernandez-Duque, a co-author of the paper, in a released statement. “I think it is important to continue communicating the idea that in our industrial societies we may not be getting all of the sleep we should.”
A public health epidemic
The agency pointed out that within an average 24-hour period, a third of American adults get less than the recommended minimum of seven hours of sleep.
Lack of sleep has been linked to traffic accidents, as well as to industrial accidents and medical errors. In fact, 5 percent of Americans admit to having nodded off while driving within the previous month — a truly scary statistic.
So get to bed earlier — and make sure you turn off the lights.
This new study is available in full online (PDF).