There’s a vague but common notion that says back in the “good old days” people used to sleep a lot longer.

Well, if your definition of the “good old days” reaches back far enough to include prehistoric hunter-gatherer societies, then we have some news… this notion of more sleep is probably wrong.

A study published in the journal Current Biology examined sleep patterns of tribes living the pre-industrial life in modern-day Tanzania, Namibia and Bolivia. On average, the study participants slept between 5.7 and 7.1 hours per night.

For comparison’s sake, the National Sleep Foundation recommends seven to nine hours of sleep nightly for adults ages 18 to 64.

The people in these tribes live without electricity or clocks, and their sleep patterns are apparently dictated by air temperature.

Members of all three tribes followed the same routine. They fell asleep a few hours after sunset, as the air began to cool down. They woke just before dawn, when the air temperature was coolest.

During the winter, the tribe members slept about one hour longer per night, compared with summer.

In case you’re wondering, few of them took naps during daylight hours, although it happened occasionally.

It’s worth noting that all three tribes in the study were located less than 20 degrees from the equator, where there’s abundant warmth and sunshine much of the year. Pre-industrial societies living in colder climates might have a different routine.

We may never know how our ancestors slept 100,000 years ago, but this paper offers some tantalizing hints.

It also inspires a question about the modern world: If nighttime temperature changes are supposed to control our sleep cycles, what does that mean for people who spend much of their lives in temperature-controlled buildings?