Our ancestors may have got less sleep than we do, a study suggests.
US researchers studied the sleeping patterns of traditional societies in Africa and South America, whose lifestyles closely resemble ancient hunter gatherers.
They monitored 98 people for 1,165 nights, and found that they slept for an average of 6.5 hours per night.
By comparison, the scientists said that most people in the US get about seven hours, according to a large sleep poll.
The new study, published in the journal Current Biology, also finds that temperature played a greater role than light in shaping sleeping patterns.
Prof Jerome Siegel, from the University of California, Los Angeles, said: "The issue is: what is the data on how sleep has changed?
"And it occurred to me that these groups, which are rapidly disappearing, give the last opportunity to really know what human sleep was like before we all created our various civilisations.
"What is absolutely clear is that they don't sleep more than we do."
No naps
From artificial lights, to late night TV, and now the ever-present glow of our smart phones, modern life is often blamed for ruining our sleep.
To put this to the test, the researchers studied the Hadza of Tanzania, the San of Namibia and the Tsimane of Bolivia, fitting their volunteers with wristwatches that monitor sleep.
"All three groups have pretty much the same sleep duration and pretty much the same timing of sleep," said Prof Siegel.
"This gives me reasonable confidence that they reflect the common human biology and they are not a function of their particular situations, which are different."
As well as discovering that the average sleep duration was six hours and 25 minutes, the researchers also found the participants very rarely took naps.
While some European documents suggested that people used to wake up for a while during the night, sleeping in two shifts, the researchers found this was not the case with the hunter gatherers.
Surprisingly, natural light did not have as big an influence as was thought.
Most people fell asleep on average 3.3 hours after sunset.
However, temperature was an important factor.
"What we saw was quite striking - that sleep is occurring during this period of falling temperature and when the temperature hits bottom, they wake up," said Prof Siegel.
"This is quite surprising."
And despite the fact that these traditional societies slept less than the National Sleep Foundation's recommended seven to nine hours a night, the researchers said they did not grumble about being tired.
Insomnia was also extremely rare - and two of the groups did not even have a word for it.
Difficult to interpret
Commenting on the research, Prof Derk-Jan Dijk, from the Surrey Sleep Research Centre at the University of Surrey, said that it was an important study but he did not agree that the data showed that our ancestors slept less than us.
"There are people in our society who don't get enough sleep, there is no doubt about it," he told BBC News.
"The question of whether we sleep that much less than so many years ago has been unanswered in ways - we need to be careful in interpreting that data."
He said that while the hunter gatherers did not fall asleep until several hours after sunset, artificial light was keeping us awake for even longer.
He explained: "We have artificial light in abundance and we have our clock-determined social commitments and the timing doesn't have anything to do with sunrise or sunset. We are to a large extent disconnected from those natural cycles.
"I think we need to re-evaluate the timing of our social schedules, including work, relative to the natural environment. Our social environment has an impact on when we decide to go to sleep and wake up.
"Also if we look at our environmental variables in the light-dark cycle in our homes and the temperature, I hope his paper will make us see how relevant are they for the timing of our behaviour."
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