The early cave dweller gets the worm.

People prone to early rising could have the Neanderthals to thank as a new study suggests that ancient genetic mutations could be responsible for modern-day sleep cycles.

A team of researchers from various American universities found that DNA passed down from two ancestral groups of modern-day humans — Neanderthals and Denisovans — may have been the progenitors of the morning person gene.

“This was really exciting to us, and not expected,” Tony Capra, a professor and co-director of the Biological and Medical Informatics program at the University of California, San Fransisco, told New Scientist.

“Neanderthals and Denisovans passed on DNA that increased our morningness, and this has been retained in modern human populations.”

Past research has linked present-day biological benefits — fertility and immunity, for instance — to the genes of Neanderthals and Denisovans. In the recent study, published Thursday in Genome Biology and Evolution, researchers wondered if our circadian rhythms evolved from our distant ancestors in the same way.

Researchers noticed the genes that became more common over time were connected to the body’s circadian clock, the 24-hour cycle that dictates the behavioral, mental and physical changes internally, the New York Times reported.

The scientists compared the genomes of three Neanderthals and one Denisovan to that of thousands of present-day humans, specifically highlighting the 246 genes responsible for regulating the circadian clock.

Overall, they found more than 1,000 unique genetic mutations throughout the sample groups, identifying those that were specific to modern-day humans or our ancestors — particularly gene variants that pertained to the body’s internal clock. From there, scientists discovered a small number of variants shared between people living today and the Neanderthals and Denisovans.

Using genome data from the UK Biobank and questionnaire responses as to whether volunteers were night owls or early birds, the researchers discovered that those with the same genetic variants as the primitive ancestors were early risers.

The scientists theorized that adapting to environmental changes was to blame for the genetic mutations to the circadian clock.

While modern humans can trace their ancestry back approximately 300,000 years to Africa, the ancestors of present-day Eurasian people migrated to Eurasia about 70,000 years ago, where the environment drastically varied depending on the time of year.

The higher latitude, seasonal changes in sunlight and weather and interbreeding with native Eurasia hominins such the Neanderthals and Denisovans likely resulted in the genetic mutations over the course of generations as humans evolved to adapt.

“Faster clocks help other species like fruit flies adapt to higher latitudes where there is higher seasonal variation in light-dark cycles and ultraviolet exposure,” Capra told New Scientist. “We think it was the same case for Homo sapiens.”