Grandparents’ earlier bedtimes may have evolved to ensure security for the family
Humans didn’t evolve in a world of deadbolts and heavy doors. Even a cave has a entrance that you might need to keep an eye on, but the average family or village probably didn’t have a sentry staying up to look for lions. Instead, security was likely assured thanks to restless grandparents and the fact that older people’s sleep schedules rarely sync up with those of younger adults.
Duke researchers built this “poorly sleeping grandparent hypothesis” around sleep data from the Hazda people of northern Tanzania. Hazda villagers will often work separately during the day, then come together to sleep at night in groups of 20 to 30 people. With no artificial lighting and simple bedding laid out on the ground near and outdoor hearth, the Hazda are thought to be a good proxy for early human societies, possibly providing insight into how our sleep needs evolved long ago. To get the details on this lifestyle, study participants were asked to wear activity monitors so that each individual’s schedule could be compared.
Sleeping in unscheduled-shifts
The pattern that emerged is likely very familiar to many of us, even if we’re not used to sleeping outdoors. Older people tended to go to bed earlier, around 8:00 pm, while younger adults stayed up later. Appropriately, older people also woke up earlier as well, but that doesn’t fully explain how this kind of sleep schedule could be a benefit to a group’s security. The other element is how restless people were during the the night— at any given moment during the night, at a little over a third of the group was dozing or alert. There were only 18 minutes in a night when every adult was completely at rest.
This suggests that a group of mixed ages could sleep without sentries because everyone was “on duty” for part of the night. The fact that older people tend to sleep and wake early may have then evolved to reinforce this system. With sleep schedules staggered by a few hours, an extended family would have very little risk of being truly unguarded throughout the night. This is all without factoring the further disruptions of young children, who would presumably add another layer of schedules to the mix (while creating new demands of their own that a grandparent might be able to help with.)
Interestingly, these overlapping but different schedules not only avoided lion-surprises, but also provided sufficient rest. Nobody complained of sleep deprivation, suggesting that while we obviously need good amounts of sleep, the idea that a good night’s rest has to be a single, uninterrupted block of time may be a modern expectation.
Source: Live-in grandparents helped human ancestors get a safer night's sleep, Popular Archaeology