Sleep in the wild

1

Did people without electric light sleep longer?

No. In three societies without it, sleep lasted 5.7 to 7.1 hours, near the low end of industrial societies.

It has been argued that electric light and screens cut sleep short from its natural length. A team from several universities in the US and South Africa fitted wrist sensors on people in three societies with no electric light. The Hadza of Tanzania and the Kalahari San are hunter-gatherers. The Tsimane of Bolivia hunt and farm.¹

The sensors did not show long nights. Sleep lasted between 5.7 and 7.1 hours, near the low end of industrial societies. The authors suggest the likeness across three groups reflects a core human sleep pattern.¹

The study counted two things. Sleep duration is the hours actually asleep. The sleep period is the span from first falling asleep to last waking, which ran from 6.9 to 8.5 hours.¹

A second team found short sleep in the Hadza too. It followed 33 volunteers and put their average sleep at 6.25 hours, shorter than in Western populations.²

Wrist sensors judge sleep from movement. The first team says its sensors were checked against sleep recordings for night sleep, and less thoroughly for naps.¹

Your watch also guesses sleep from signals such as movement. Its totals are only a rough comparison with these.¹

2

When did they go to sleep?

Not at sunset. On average sleep began about 3.3 hours after it, and people usually woke before sunrise.

None of the three groups went to sleep near sunset. Across the groups, sleep began between 2.5 and 4.4 hours after it. Awakening usually came before sunrise.¹

Falling asleep was the flexible end of the day. Day to day, how long people slept followed when they fell asleep, not when they woke. In the San, the shorter sleep of summer came entirely from falling asleep later.¹

Light stayed low all night. The groups often had small fires, but wrist readings stayed below 5 lux, the lowest the sensors can read.¹

Sleep did line up with temperature. It began as the air cooled and ended near the coldest point of the night. The authors suggest that this daily temperature swing, largely missing from modern bedrooms, may be a strong natural regulator of sleep.¹

The study saw sleep and temperature move together. It did not test cooling a bedroom, so that part is an idea, not a result.¹

If your bedroom sits at one temperature from evening to morning, it lacks the swing these sleepers had.¹

3

Does electric light cut sleep short?

Sometimes. Two studies found 30 to 56 minutes less sleep with electric light, one found none, and all saw later bedtimes.

Two Toba/Qom communities in northern Argentina share a history, but one has electric light and the other lives by natural light only. Participants wore wrist loggers for a week in summer and a week in winter. The community with electricity slept 56 minutes less in winter, and showed a tendency toward 43 minutes less in summer.³

The gap sat at the evening end. In both seasons, bedtimes in the community with electricity were later, while wake times were similar.³

The groups were small, from 6 to 11 people each. The community without electricity also tended to be younger, by more than a decade on average in summer, and it took part more in foraging.³

In a questionnaire survey, Amazon rubber tappers with electric light at home reported 30 minutes less sleep on work days than those without it.⁴

Mozambique gave a twist. In a town and a neighbouring village, bedtimes were about an hour later in the town, with more evening and less daytime light. Yet sleep lasted about 7.3 hours in both.⁵

If an alarm fixes your wake time, a later bedtime is where the hours go.³

4

Does the season change sleep?

Yes. The Tsimane and San slept nearly an hour longer in winter, and the Toba/Qom slept longer too.

The Tsimane and San live far enough south of the equator for day length and temperature to change a lot with the seasons. Tsimane people recorded in winter slept 56 minutes longer than those recorded in summer. The same 13 San people, recorded in both seasons, slept 53 minutes longer in winter.¹

The two Toba/Qom communities did the same: both slept longer in winter than in summer.³

For the Tsimane, summer and winter were different groups of people, and for the Toba/Qom nearly so. Only the San were recorded in both seasons.^1,3

If your own sleep creeps longer in the dark months, that fits what these groups did.¹

5

Did they sleep well, and how sure can we be?

Few reported trouble sleeping, but two studies disagreed on naps and sleep efficiency.

Neither the Tsimane nor the San has a word for insomnia, so the team explained it in plain terms. In interviews, 1.5% said they regularly struggled to fall asleep and 2.5% to stay asleep, which the team counted as more than once a year. The authors call that far lower than the 10 to 30% chronic insomnia reported in industrial societies.¹

The second study, on the Hadza, found a different picture. Its volunteers had poorer sleep quality than Western populations, with about 69% sleep efficiency, the share of time in bed spent asleep. Naps showed up on 54% of days.²

The first study had put sleep efficiency at 81 to 86%, similar to industrial populations.¹

Its nap counts, up to 7% of winter days and 22% of summer days, are for the San, and it calls them a maximum. Its records of the Hadza and Tsimane also showed few naps. Wrist sensors are less well checked on naps, and the people studied differed, so the gap between the studies stays unexplained.¹

Mozambique differed in quality too, in the town's favour. The village sleepers had poorer calculated sleep quality than the town residents. Poor quality went with harder physical labour and less comfortable sleeping arrangements.⁵

Your watch's sleep efficiency and nap count are estimates from signals such as movement. Two studies of foraging groups disagreed on them, so read yours as a rough guide.^1,2

6

What does this mean for you?

Sleep sat at the low end of industrial figures. Evening light may matter, but none of it shows what you need.

1. Eight hours. These groups slept less than eight hours, but that is not a verdict on how long you need. The sleep-need article covers that.¹
2. The evening end. Communities with electric light had later bedtimes. Sleep was shorter in the Argentine and Amazon comparisons, not in Mozambique.^3,4,5
3. A cooler night. Sleep here began as the air cooled. The study did not test a cooler bedroom.¹
4. Trouble sleeping? These studies describe groups. If poor sleep bothers you, ask your doctor.