Now I lay me down to sleep

ELECTRIC lighting, television, the internet and caffeine all get blamed for reducing the amount of time people sleep compared with the days before such luxuries existed. Such alleged sleep deprivation is sometimes held responsible for a rise of obesity, mood disorders and other modern ailments. The trouble with this argument, though, is that no one really knows how long people slept before coffee and light bulbs existed.

A study just published in Current Biology by Jerome Siegel of the University of California, Los Angeles and Gandhi Yetish of the University of New Mexico tries to provide an answer. Dr Siegel and Mr Yetish looked at three groups who cleave to pre-industrial (indeed, pre-agricultural) ways of life, to see if their sleep patterns differ from those of wired, urban humanity. To some surprise, they have found that in many ways they do not.

The groups in question are the Hadza of northern Tanzania, the Ju/’hoansi San of the Kalahari Desert, in southern Africa, and the Tsimané in Bolivia. All live largely by hunting and gathering. Dr Siegel and Mr Yetish asked for volunteers, and 94 people agreed to collaborate with them by wearing devices that recorded their level of movement, and also when the blood vessels near their skin were constricting. (This happens when people who do not live in environments managed by thermostats wake up: it makes more blood available to the brain and other internal organs.) Dr Siegel and Mr Yetish also put humidity- and temperature-monitoring devices in the areas where their volunteers tended to rest at night, in order to find out if these variables helped determine when they went to sleep and woke up.

In total, the researchers collected 1,165 days’ worth of data. They found that people from all three groups slept for between 5.7 and 7.1 hours a day, with an average that hovered around 6.5 hours. Far from exceeding those of a modern city-dweller, these values are near the low end of the range found in industrial societies. An average 7.5 hours a night is the norm there.

Nor did the Hadza, the Ju/’hoansi San or the Tsimané retire shortly after the sun went down. Rather, they stayed awake for an average of 3.3 hours after nightfall, much as people in the developed world do. Their bedtimes appeared to be regulated by the temperature, rather than by daylight, and it takes several hours after the sun has set for things to cool down.

The study also calls into question the idea that siestas are a feature of human behaviour that has been suppressed by modern ways of life. The volunteers rarely napped in summer (doing so on about one day in five), and almost never in winter.

There were some differences. One was that hunter-gatherers exhibited a bigger seasonal variation in the amount of sleep they took than “modern” folk do. They slept almost an hour longer in winter than in summer, whereas the wired sleep about 20 minutes longer. Perhaps more intriguingly, they barely suffered from insomnia, a complaint prevalent in more than 20% of the population of industrial societies.

Whether those who hunt and gather have anything to teach the modern world, then, is moot. A closer examination of their lives may reveal ways of reducing insomnia. But the next time someone claims a siesta is a natural part of human life, rather than a response to partying after midnight, be sceptical.