In cultures without electricity, people consistently go to bed 3 to 3.5 hours after sunset with little variability in sleep onset.

“What I found was most of these Argentinian Towers who have no access to electricity they consistently go to bed somewhere between three to three and a half hours after Sunset so this is very important because we always think that um our ancestors when they didn't have electricity as soon as the sun went down they just went to sleep no the fire extended the evening so they were staying awake for three to four hours kind of um you know decompressing themselves that we say and then doing all these activities cooking sharing meals and then they would go to sleep and if you look at the slip onset variability it was very small like they're going to bed almost within 15 to 30 minutes standard deviation”