Did Electric Light Shorten Sleep? Four Field Studies, Compared

Two Toba/Qom communities in northern Argentina sleep different amounts, and the difference is an electric wire: in a 2015 comparison, the community with electricity slept roughly 40 to 60 minutes less, most of it lost at bedtime rather than at the alarm. Where researchers fitted actigraph watches to 94 people living without electricity, the average night was 6.4 hours (Yetish et al., Current Biology, 2015), not the nine or ten that the popular story assumes.

That is a smaller claim than the one most people have heard. The common version says that before the light bulb humans slept nine or ten hours, that Edison cut it to seven, and that the missing time explains modern tiredness. Between 2015 and 2019, four research groups put actigraph watches on people living with and without electric light. None of them found the nine-hour sleeper. Here is what each measured, how the results line up, and where they stop.

What did Yetish and colleagues measure in Africa and Bolivia?

The study most often cited is “Natural Sleep and Its Seasonal Variations in Three Pre-industrial Societies,” published in Current Biology in November 2015. Gandhi Yetish led it with Hillard Kaplan, Michael Gurven, Brian Wood, Herman Pontzer, Paul Manger, Christopher Wilson, Ronald McGregor and Jerome Siegel. The group fitted actigraphy watches on 94 people in three places: the Hadza of Tanzania, the San of Namibia and the Tsimane of Bolivia. The recordings added up to 1,165 days.

The average sleep period was 6.4 hours, with the three groups ranging from 5.7 to 7.1 hours. Sleep began about 3.3 hours after sunset, and the waking time was far more regular than the going-to-sleep time. Napping was infrequent. Winter nights ran longer than summer ones by 56 minutes among the Tsimane and 53 among the San.

Two further findings matter for the folk story. When the researchers asked about sleep trouble, only 1.5 to 2.5 percent of participants reported difficulty falling or staying asleep, compared with the 10 to 30 percent usually reported in industrialized countries. And the records showed no segmented sleep pattern. The authors read the results as a challenge to the idea that industrial life has sharply cut sleep.

Wrist actigraphy infers sleep from movement and tends to count quiet wakefulness as sleep, so a number like 6.4 hours is likely an overestimate of actual sleep within a recorded period. All three groups live in tropical or subtropical settings where day length changes modestly across the year, so the findings say little about a winter at 60 degrees north. And these are three small communities whose members spend their days walking, foraging and farming, which differs from a desk worker in a lit apartment in more ways than lighting.

Is 6.4 hours a stable figure, or does it move with the community?

A second Hadza dataset complicates the picture. David Samson of the University of Toronto, with Alyssa Crittenden, Ibrahim Mabulla, Audax Mabulla and Charles Nunn, published “Hadza sleep biology: Evidence for flexible sleep-wake patterns in hunter-gatherers” in the American Journal of Physical Anthropology in 2017. They tracked 33 volunteers for 393 days in total. The Hadza averaged 6.25 hours of nighttime sleep with a sleep efficiency of 68.9 percent, shorter and more broken than Western norms, though their daily rhythms were stronger.

What stands out is how much sleep moved with conditions. Duration fell with more activity, older age and more light, and varied with moon phase, though the direction of the moon effect is not something to lean on here. It rose with longer days and warmer nights. Samson’s group also documented opportunistic daytime napping, which Yetish’s team had seen only rarely, and described the Hadza schedule as flexible, anchored by a consistent early-morning waking.

Read together, the two papers say that the Hadza and their neighbors sleep a little under 6.5 hours on a recorded night, with the exact figure depending on the season, the camp and who is counted. Neither supports a hidden eight-to-nine-hour baseline. A wider tour of how sleep differs across countries touches the Hadza briefly. The sharper question is what happens when a population gets a light switch.

What happened when a Toba/Qom community got electricity?

The cleanest comparison comes from northeastern Argentina. Horacio de la Iglesia led the team, with Eduardo Fernandez-Duque, Diego Golombek, Norberto Lanza, Jeanne Duffy, Charles Czeisler and Claudia Valeggia, an anthropologist at Yale. Their paper, “Access to Electric Light Is Associated with Shorter Sleep Duration in a Traditionally Hunter-Gatherer Community,” appeared in the Journal of Biological Rhythms (volume 30, issue 4, 2015).

The Toba/Qom are a historically hunter-gatherer people of the Argentine Chaco. The team compared two nearby communities with the same ethnic and economic background: one on the edge of a small town with ready access to electricity, and another that used only natural light. Participants wore actigraphs in both summer and winter, during fieldwork in 2012 and 2013.

In the community with electricity, bedtimes were later and more variable, while both groups got up at about the same time. In summer, the electrified group slept about 43 minutes less, which the authors reported as a tendency. In winter the difference was 56 minutes, the clearer result. The authors did not claim that the light bulb caused the whole gap. They proposed that evening light delays the biological clock and later tested the idea with melatonin, reporting a delay in dim-light melatonin onset in the community with electric light.

This study supports the reasonable core of the popular story and sets the scale: close to an hour, not two or three. It also carries the limits that apply to the whole field. Only two communities were compared, so people living near the town may differ in work, diet or family structure, and the samples were small. A 2025 preprint on the Qom from some of the same researchers, “Modern Times: Longitudinal Study of Toba/Qom Communities Reveals Delay and Shortening of Sleep in Real-Time Across Electrification,” follows participants across years of electrification, which addresses the cross-sectional weakness. It reports that rural participants lost about an hour of sleep within a decade, and that urban changes suggest phones and internet matter beyond light. It has not been peer reviewed, so treat its numbers as provisional.

Does the effect show up on other continents?

A 2019 paper in Scientific Reports by Andrea N. Smit, Tanya Broesch, Jerome M. Siegel and Ralph E. Mistlberger tested the idea on Tanna Island, Vanuatu, comparing villages with and without electric lighting by actigraphy. In electrified villages, light exposure after sunset was greater, sleep started later and nightly duration was shorter. The authors reported that the effect was driven mainly by breastfeeding mothers living with electric lighting, so the finding is narrower than a village-wide one.

So the shortening looks real but uneven, concentrated among people who are awake in the evening for other reasons and have a lamp to do it by.

Quick answers on lit and unlit sleep

How many hours of sleep do adults need, and where do the field averages fall? The American Academy of Sleep Medicine and Sleep Research Society consensus statement (Watson et al., 2015) recommends at least seven hours a night for adults. The field averages of 6.4 and 6.25 hours sit below that line, but they were measured by wrist actigraphy in communities with very different daytime activity, so the two figures are not directly comparable.

Did the Hadza, San and Tsimane show a first sleep and a second sleep? No. Yetish and colleagues reported that sleep in all three groups was consolidated into one nightly period, with little napping and no segmented or bimodal pattern. That does not disprove Roger Ekirch’s historical record from preindustrial Europe, because the two datasets come from different places, centuries and climates.

What is actigraphy, and why do these studies use it? Actigraphy uses a wrist-worn motion sensor to infer sleep and wake from movement. It works in villages and camps where an overnight laboratory study is impractical. It estimates sleep from movement rather than brain activity, so a person lying still but awake can be counted as asleep, and the 6.4-hour figure is a sleep-period estimate, not an EEG measurement.

Where does the segmented-sleep story fit?

Historian Roger Ekirch of Virginia Tech spent years collecting references to “first sleep” and “second sleep” in diaries, court records and medical texts, and published them in At Day’s Close: Night in Times Past in 2005. His claim is that in preindustrial Europe, people commonly slept in two blocks with an hour or so of wakefulness between them. The history of that two-part night is already laid out elsewhere on this site.

It does not clash directly with the field data. Ekirch describes northern European households, where winter nights run long, and his sources are texts, not wrist devices. Yetish’s participants lived near the equator or in the southern subtropics, in huts and shelters without winter nights of that length. A sensible reading is that long dark nights can produce a broken sleep, and that short ones usually do not. The Hadza, San and Tsimane data neither confirm nor refute Ekirch; they show that a segmented night is not a universal pre-electric habit, which is a narrower claim than the one sometimes made on his behalf.

What does a week in a tent add?

Kenneth Wright’s group at the University of Colorado Boulder supplies the nearest thing to an experiment. In a 2013 Current Biology paper, Wright and colleagues sent eight adults camping in the Rocky Mountains for a week with no electric light, only sunshine and campfires. The onset of their melatonin secretion moved about two hours earlier, and their sleep shifted more than an hour earlier. Follow-up work in the same journal found that a summer weekend moved melatonin onset 1.4 hours earlier, and that five people camping for a week near the winter solstice shifted by 2.6 hours.

These samples are small. Still, the results match the field studies on direction and rough scale: remove evening light and the internal clock moves earlier by one to a few hours, within days. The post on how light sets the body clock covers why.

Is an alarm clock a kind of artificial sunrise?

In the electrified Toba/Qom community, waking time stayed about the same as in the unlit one, and bedtime slid later. An alarm clock works as an artificial sunrise: it pins the wake end of the night in place. Anything that delays the clock in the evening, whether lamps, a bright screen or a late social life, therefore subtracts directly from sleep, because there is no slack at the other end.

Wright’s camping data indicate that dimmer evenings and brighter days move the body clock earlier within days, the direction a person with an early alarm needs. The effect is probably smaller for someone in a lit city than in a tent, and none of the studies say how much dimming lamps at home would help. Nor do they say that 6.4 hours is the right amount for a person with a commute and a screen; they describe sleep in particular places. A person on a fixed wake time who feels short of sleep should not conclude that modern life has changed their biology. They can look at when their evening light and their bedtime actually fall, and at whether the gap is closer to ten minutes or an hour. Regularity of timing has its own research behind it, and it points the same way: the clock keeps better time when its end points stop moving.