A new research paper finds that the genetic material of Neanderthal ancestors may have contributed to the tendency of some people today to be “early risers” — people who wake up earlier and sleep more soundly. The findings were published in the journal Genome Biology and Evolution.

Anatomically modern humans all appeared in Africa about 300,000 years ago, where environmental factors shaped many of their biological characteristics. About 70,000 years ago, ancestors of modern Eurasian humans began migrating into Eurasia; Here they encountered a variety of new environments, including higher latitudes with greater seasonal variation in daylight and temperature.

But other hominins, such as Neanderthals and Denisovans, lived in Eurasia for more than 400,000 years. These archaic hominins diverged from modern humans about 700,000 years ago, and as a result, our ancestors and archaic hominins evolved in different environmental conditions. This resulted in the accumulation of lineage-specific genetic variation and phenotypes. When humans arrived in Eurasia, they interbred with archaic hominids on the continent, creating the potential for humans to acquire genetic variants already adapted to these new environments.

Previous studies had shown that most archaic hominin ancestors in modern humans were not beneficial and were eliminated through natural selection, but some archaic hominin variants that remained in human populations showed signs of adaptation. For example, archaic genetic variants have been associated with differences in high-altitude hemoglobin levels, immune resistance to novel pathogens, skin pigmentation levels, and fat composition in Tibetans.

Changes in structure and light levels have biological and behavioral consequences that can lead to evolutionary adaptations. Scientists have previously extensively studied the evolution of circadian adaptation in insects, plants and fish, but it remains poorly understood in humans.

The Eurasian environment where Neanderthals and Denisovans lived for several hundred thousand years was located at higher latitudes with greater daylight diversity than the region where modern humans evolved before leaving Africa. So the researchers investigated whether there was genetic evidence for differences in the circadian clocks of Neanderthals and modern humans.

The researchers identified 246 circadian gene clusters through a combination of literature searches and expert knowledge. They found hundreds of genetic variants unique to each line that could potentially affect genes involved in the circadian clock. Using artificial intelligence techniques, they identified 28 circadian genes that contained variants with the potential to alter splicing in archaic humans and 16 circadian genes that were likely to be differentially regulated between modern humans and archaic hominins.

This suggests that there are likely functional differences between the circadian clocks of archaic hominids and modern humans. Because the ancestors of modern Eurasian humans interbred with the ancestors of Neanderthals, it is possible that some humans inherited circadian variants from Neanderthals.

To test this, researchers examined whether introgressed genetic variants (variants passed from Neanderthals to modern humans) were associated with the body’s wakefulness and sleep preferences in a large cohort of several hundred thousand people in the UK Biobank.

They found many introgressive variables that affected sleep preference, and most strikingly, they found that these variables consistently increased “moriliness,” the tendency to wake up early. This suggests a directional effect on the trait and is consistent with high-latitude adaptations observed in other animals.

In humans, the increase in morning level is associated with a shortened period of the circadian clock. This is likely beneficial at higher latitudes as it has been shown to allow faster sleep/wake adaptation with external time signatures. In fruit flies, circadian periods must be shortened to synchronize with long summer light periods at high latitudes, and selection for shorter circadian periods has led to latitudinal gradients of decreasing periods with increasing latitude in natural fruit fly populations.

Therefore, morning bias in introgressive variants may be indicative of selection towards a shortened circadian period in high-latitude populations. The tendency to be morning people may have been evolutionarily beneficial to our ancestors living in the high latitudes of Europe, and thus may have been a genetic trait of Neanderthals worth preserving.