Sleep pattern variation across cultures caused by evolution

Sleep patterns in humans today are very different today from what they were a hundred years ago. They have continued to change throughout history, in both quantity and quality. Studying sleeping patterns across cultures over time is important to understanding the effects sleep has on human health. Thus, researchers are looking to figure out whether this change in sleep was brought about by socioeconomic, technological and political developments or if it is a product of evolutionary mismatch in early human beings’ sleeping patterns.

A paper published in the American Journal of Physical Anthropology studied cross-cultural sleeping patterns in an attempt to address this question.

The study conducted by David Samson of Duke University, and his team analyzed the patterns in 33 volunteers of the Hadza hunter-gatherer community in Tanzania over a period of 393 days.

Such societies are typically under-represented in sleep research studies which predominantly comprise of western, educated, industrialized, rich and democratic social groups. The Hazda community represents a small-scale, non-industrialized population that doesn’t have environmental barriers such as air-conditioners, heaters, or sound-proofed rooms that would affect sleep.

The researchers used wrist devices to gather information and found that the subjects slept for an average of 6.5 hours per night despite spending a longer time in bed which averages around 9.16 hours per night.

Using a scalp device, on nine participants, the researchers also found that the villagers experienced shorter slow-wave cycles and rapid-eye movement which suggests a biologically relatively light sleep.

As compared to western sleeping patterns, the Hadza community seemed to have shorter and poorer quality of sleep. Besides that, they also took frequent daytime naps for an average of 47.5 minutes. Despite that, Hazdas seemed to have stronger circadian rhythms.

According to an article about the study in Science News, westerners about 500 to 200 years ago had similar sleeping patterns, i.e., two distinct spans of sleep in one day. Thus, this population has a lower sleep efficiency as compared to western, industrialized populations.

Many other sleep research studies conclude that irregular sleep cycles and lack of exposure to sunlight increases risk of heart disease and other health issues. Besides that, use of electronic devices with blue-screens such as mobile phones, laptops, televisions etc. after dark negatively affects melatonin production which delays sleep.

Furthermore, the study concluded that environmental factors such as light and temperature significantly impacted sleep duration and quality. Increased ambient light and temperatures at night were found to increase sleep quality. This observation was interpreted in the paper to mean that experiencing longer days (equivalently more ambient light) and tropical temperatures, implied experiencing longer sleep duration.

Based on other interpretations of the data they collected, the researchers lean towards concluding that sleep plasticity, i.e., variation in sleep quality and quantity has been a product of natural selection in human evolution.