Early birds, night owls and wakeful elderly people – why we sleep when we do, by Melissa Cunningham.
By tracking the sleep activity of 22 modern-day Hadza hunter-gatherers in Tanzania, researchers from the Royal Society found over a period of 20 days that there were only 18 minutes when no one was awake.
The scientists believe the strategy explains why there remains a wide variety in the sleeping habits of modern-day humans, including elderly people more likely to be awake than youngsters.
While sleep is essential for human survival, for our Paleolithic ancestors it also represented a time of extreme vulnerability.
The researchers proposed that group-living animals shared the task of vigilance during rest periods. In other words, somebody must always be awake to protect the tribe from dangers such as the jaws, horns or claws of wild beasts.
One of the scientists leading the study and a senior researcher at Duke University, Dr David Samson, says the variation in sleep patterns and higher rates of wakeful older individuals in modern humans may be a legacy of natural selection acting to reduce the dangers of sleep. …
Dr Samson said the results showed one or more individuals were awake during 99.8 per cent of sampled time periods between when the first person went to sleep and when the last person awoke.