A new study published December 14th in the journal Cell Reports provides evidence that variants of certain genes inherited from our Neanderthal ancestors may explain why some people rise early in the morning while others prefer to stay up late at night.
Key Points
- Researchers found that genetic variants associated with early rising are more common in regions of the human genome known to contain Neanderthal DNA segments.
- These variants likely regulated our ancient human relatives’ circadian rhythms, influencing when they felt sleepy or awake.
- Inheriting this “early riser” DNA may have given some human populations an advantage in coordinating hunting, gathering, or other tasks with daily cycles.
- The findings illustrate how traces of Neanderthal DNA still shape modern human health and behavior.
Tracing Ancestral Circadian Rhythms
All humans share a roughly 24-hour circadian rhythm that influences key bodily processes and behaviors, including the sleep-wake cycle. This internal clock is known to be regulated by more than 300 genes.
To pinpoint genetic variants associated with being an early riser, defined as waking up before 6 AM consistently, the researchers analyzed genomic and self-reported sleep data from nearly 700,000 UK Biobank study participants of European ancestry.
They identified 224 gene variants linked to morning preference, including in MEIS1, a gene important for development of the pacemaker neurons that control circadian rhythm. Crucially, these early bird-associated variants were more concentrated in regions of the human genome previously identified as containing DNA inherited from Neanderthals.
Analyzing public DNA databases, the scientists confirmed that early rising variants were depleted among African populations with little or no Neanderthal DNA. Such populations instead had more DNA variants keeping in tune with a night owl chronotype.
Early rising DNA variants are more common in Europeans with Neanderthal DNA segments (Han Chinese shown for comparison). Image credit: Hu et al., Sci Adv 2022, adapted under CC BY 4.0 (source)
Neanderthals Were Early To Bed, Early To Rise
So why would inheriting Neanderthal genes make some people prone to waking before dawn? The answer may lie in examining what experts know about how these extinct hominins lived:
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Archaeological evidence shows Neanderthals mostly occupied higher latitude Eurasian regions. Here, large seasonal differences in daylight hours would have exerted evolutionary pressure to entrain (synchronize) their internal clocks with environmental light-dark cycles.
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As specialized big game hunters living in small bands, Neanderthals likely had to coordinate timing of sleep and wakefulness among group members for cooperative activities like nighttime hunting or morning processing of kills.
Table: Putative differences in sleep habits of Neanderthals vs. contemporary humans
| Characteristic | Neanderthals | Modern humans |
|---|---|---|
| Typical latitudes occupied | Higher latitudes | Wider range of latitudes |
| Importance of seasonal/daily environmental cues | High | Lower |
| Social structure | Small cooperative bands | More flexible arrangements |
| Typical sleep period | Shorter, aligned with available nighttime light | Longer, more flexible timing |
- Comparisons of Neanderthal and contemporary human skeletons suggest our extinct cousins may have had shorter average sleep requirements. Having a sleep-wake schedule attuned to natural light cycles would have suited their high-latitude, highly seasonal habitat.
In contrast, anatomically modern humans evolving in Africa would have faced less pressure to rigidly regulate their internal clocks. With smaller daily/annual light fluctuations nearer the equator, a more flexible chronotype would have been feasible.
Early Riser DNA: An Evolutionary Advantage
For early European humans with some Neanderthal ancestry, inheriting their relatives’ early-to-rise chronotype may have provided benefits that increased survival and reproduction.
Some possibilities include:
- Better coordination of hunting, gathering, migrating, or other tasks among group members.
- Aligning sleep more closely with fluctuations in temperature, light, and other environmental conditions.
- Avoiding the coldest nighttime temperatures during Ice Age climactic conditions.
Over successive generations, natural selection would have favored and spread early rising DNA variants within these human populations. That may explain why Neanderthal variants associate with morningness at certain genes like MEIS1 persist in around half of all living Europeans today.
Ongoing Clues To Our Complex Shared History
These new insights show how the legacy of genetic intermixing between early modern humans and Neanderthals continues shaping human health and behavior today. They also underscore the value of recent advances in analyzing ancient DNA.
Professor Arnold Nalesnik, an evolutionary anthropologist not involved with the current study, said: “This research highlights that we are really just beginning to unravel the genetic complexity behind circadian rhythms. As more Neanderthal DNA is sequenced, we may discover additional variants they passed down that adapted our human ancestors to Ice Age environmental conditions in Eurasia.”
“On a broader level,” Nalesnik continued, “these findings remind us that human evolution has always been a tale of migration, adaptation and genetic exchange between different groups over time. Appreciating this shared history can help us understand and celebrate both our commonalities as one human species as well as our diversity.”
What’s Next For Circadian Genetics
Looking ahead, the researchers behind this latest study want to clarify exactly how Neanderthal DNA alterations affect activity levels of specific genes influencing circadian rhythm, including across different tissue types.
Integrating more insights about ancient hominin ecology will also help evolutionary geneticists piece together what sleep and wakefulness patterns may have looked, and felt, like for different human species over hundreds of thousands of years.
On a practical level, better grasping this evolutionary history may someday aid diagnosing and treating sleep disorders, which impact millions worldwide. It could also allow more personalized chronotherapeutic interventions tailored to a person’s ancestral background and circadian genotype.
So while the case is not completely closed, we may need to at least partially thank remnants of Neanderthal DNA within our own genomes if we find ourselves rising bright and early every morning.
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