Everybody loves sleeping in. A new study, however, warns that too much sleep can actually have a negative effect on your alertness throughout the course of the day. According to a report from the Boston Globe, a study of some of the planet’s last remaining hunter-gatherer tribes has revealed that following the rhythms of the sun to guide our sleep patterns may not be the way our ancestors actually slept.

The study, published in the journal Current Biology, examined the Hadza people in Tanzania, the San people in Nambia, and the Tsimane in Bolivia. These populations have had little contact with industrial societies, and thus follow the sleep schedule that suits them best.

First, the researchers studied the Hazda. This group lives in a woodland savannah environment roughly two degrees below the equator. They do not roam far from their home, remaining isolated from surrounding villages and live largely off of food they can gather on their own.

The San, who live on the Atlantic coast of southern Africa, are similarly isolated. Unlike the Hazda, however, they grow a good deal of their own food. The researchers used Actiwatch-2 devices to monitor the sleeping habits of 94 individuals over a study period of roughly 1,165 days. The device is similar to a medical-grade Fitbit sleep monitor.

Despite the wide distances between the Hazda, the Tsimane, and the San, the researchers found their sleep patterns to be strikingly similar. The three groups got about 6.5 hours of sleep each night on average. They did not take naps and they didn’t fall asleep right when it got dark.

The Hazda, San, and Tsimane stayed up in bed for a few hours before falling asleep, much like many modern industrialized populations. This is equivalent to a sleep efficiency of roughly 86 percent, about what it is in the United States.