Among the hunters and collectors, old and young people lived together with different sleep-times, so that they never slept at the same time at night

Man sleeping outdoors from the people of Hazda

Durham (USA) - The daily sleep-wake rhythm changes in the course of life: Younger people are usually active until the late hours of the night and are still tired in the early morning. The older ones, on the other hand, tend to sleep early and often wake up before daybreak. The fact that there are so different "chronotypes" could have proven to be beneficial in human evolution, is now reported by an international team of anthropologists. The people of the Hadza, who still live as hunters and collectors, have seen very different sleeping times among the members of a group. During the night, at least one was awake and ready to warn the others of possible dangers, the researchers write in the "Proceedings of the Royal Society B". The guardian function of night owls, early risers, and troubled sleepers may have allowed early humans to benefit from the positive effects of adequate sleep on memory and learning ability.

"Many elderly people complain that they often wake up in the night and can not fall asleep again," says Charles Nunn of Duke University in Durham, one of the leading scientists. "Perhaps we should not consider this as a sleep disorder, but as a relic from an earlier phase of our evolution, in which this behavior was an advantage." The sleep, which is vital for humans and animals, increases the risk of becoming prey to predators or menacing Environmental impacts. According to the so-called guardian hypothesis, animals living in groups share the task of being alert during sleep: a few remain alert and report a threat so that others can sleep without danger.

Nunn and his colleagues wanted to find out how people in ancient times could have protected themselves from nightly dangers. They documented the sleep-wake rhythm of the Hadza, which are in groups of 20 to 30 people. Their traditional way of life in Tanzania is probably similar to the living conditions of the people before they became sedentary. 33 men and women between the ages of 20 and 60 agreed to participate in the investigation. An actometer, which is worn like a wristwatch, recorded all movements during the night for 20 days. The day's rhythm of the Hadza is not yet influenced by artificial lighting and other modern technology.

The subjects were sleeping on average at 10 pm and woke up at 7 o'clock. The sleep behavior of each individual within a group was however not synchronized at all, but individually very different. Some also woke up after sleeping, and left their sleeping places temporarily. During the total of 220 hours of the recorded monitoring time, there were only 18 episodes of one minute each of the group members slept at the same time. At every night, an average of one third of the group was active or in a light sleep. This share would increase even if patients and children had been included. No one complained about sleep problems.

The results explain why the Hadza usually do not need guardians for their night-time camps: this is simply not necessary because someone is always awake anyway. The researchers call their new theory "Hypothesis of the badly sleeping grandparents". According to her, especially the cohabitation with the older generation, which often has a temporally advanced, shorter and interrupted sleep, increases the security of the group at night. Therefore, such groups of people might once have had a survival advantage, the members of which differ greatly in their sleep-wake rhythms. In the case of Hadza, this principle works only if the group has a minimum size of about twelve persons. For example, if South Africans are on the road for several days, the authors say that they must keep a watch at night.