Onsite in Bolivia
Tsimane translator (l) with Ghandi Yetish.
Credit: Courtesy of Ghandi Yetish

“They go to sleep several hours after sunset and typically awaken before sunrise.” That’s one of the main conclusions in a new paper published in “Current Biology.” UNM doctoral candidate Gandi Yetish is the lead author in the paper "Natural Sleep and its Seasonal Variations in Three Pre-industrial Societies."

Typical Tsimane home
Typical Tsimane home

Researchers examined the sleep patterns of three societies of hunter-gatherers; the Hadza in northern Tanzania, south of the equator, the Kalahari San in northeast Namibia, and the Tsimane who live close to the Maniqui River in Bolivia.  The San are not currently nomadic, but live in villages as hunter gatherers.

Yetish says they found that these hunter-gatherers and hunter-horticulturalists sleep an average of 6.4 hours a day, rarely nap and all show similar sleep amounts and timing. The research concludes that temperature seems to regulate sleep duration and timing since the members of the groups went to sleep as daytime temperatures began to drop and awoke when nighttime temperatures were at their lowest. The researchers could not find a tight link to ambient light levels and sleep patterns.

The findings will be presented at the Neuroscience 2015 conference this weekend.

Yetish is a student of UNM Professor of Anthropology Hilliard Kaplan, who has studied the Tsimane in Bolivia for decades. Yetish went to Bolivia in 2013 with a specific interest in studying when people should want to sleep more, and when they should want to sleep less and why. While in Bolivia he was contacted by University of California Neurobiology Research Professor Jerome Siegel who wanted to coordinate sleep research among several hunter-gatherer groups in different parts of the world.

Yetish says this has been a great learning experience for him. “I’m hoping to continue my research into the evolution of human sleep and its ecological context to inform the way we think about our own sleep in the U.S.,” he said.

View of Bolivian jungle and river from airplane
View of Bolivian jungle and river from airplane

Individuals in the societies they studied tend to have lower blood pressure and atherosclerosis and higher levels of physical fitness than is normal in industrialized populations. The research showed that the Tsimane and San participants in the study tended to sleep nearly an hour longer in the winter than they did during the summer.

The study participants reported few sleep problems. About 5 percent reported some trouble falling asleep and about 9 percent reported trouble staying asleep. That is comparted to the ten to 30 percent of chronic insomnia problems reported in industrial societies.

The paper was published cooperatively with researchers from the UNM Department of Anthropology, the University of California, Santa Barbara, Yale University, Hunter College, the University of the Witwatersrand in Johannesburg, South Africa, the University of California Los Angeles, the VA Greater Los Angeles Healthcare System, and the Brain Research Institute at UCLA.