Grandparents typically nod off earlier than everyone else in the family, but not for the reason you might think. According to an anthropological study of Hadza hunter-gatherers in Tanzania, the elderly snooze sooner because of a survival mechanism left over from natural selection.
The team of Canadian, American, and Tanzanian researchers observed that after a day of gathering food, the tribe would meet in the same spot to rest. Not so smart if everyone’s snoozing and an environmental or predatory threat comes along in the night.
Someone needs to burn the midnight oil and keep guard in case the group needs to be alerted, so the “sleep architecture” of the population ensures someone is awake at every point. The aptly-titled “poorly sleeping grandparent hypothesis” argues that sleeping in mixed-age groups aided our ancestors’ survival because different age groups nod off at different times.
Actigraphy – a non-invasive measurement of gross motor movement – was the method used to calculate rest/activity cycles of the tribal members. The device used is called an actigraph, and the subject typically places it on their wrist like a watch.
During the 20-day observation period, the data showed that the whole tribe was asleep for only 18 minutes in total. This translates to at least one person being awake for 99.8% of the time between the first snoozer and the last to rise.
Subjects in their 50s and 60s went to sleep earlier and got up sooner than their younger counterparts. Less robust bodies do get tired quicker, of course, so it makes sense that the young ones are first on duty.
For modern populations, this would explain a lot about the spectrum of sleeping patterns in a family. The passing of the ancestral baton doesn’t stop there, however, as evolutionary hangovers such as this can be everything from hilarious to downright terrifying.
Dean Burnett’s The Idiot Brain is a must-read for those interested in the calamity of inherited behaviours. Burnett, an English neuroscientist and author, explores phenomena such as the prevalence of conspiracy theorists and over-confidence in less intelligent people.
Having a copy handy might help you entertain yourself the next time your grandparents fall asleep on the couch.