Primates, including humans, mostly give birth during their least active time of day. For us, that’s during the night but exactly why most babies come into the world around this time has been unclear. Now, a new study looking at Vervet monkeys has found that there may be a link to body temperature, with the birthing window coinciding with when primates’ bodies are at their coolest.

The researchers behind the new paper, published in Biology Letters, looked to wild Vervet monkeys (Chlorocebus pygerythrus) for insights into birthing patterns. During their observation period, they recorded the body temperature of 24 females as they gave birth to get a better idea of what was going on.

Their data revealed that the mothers’ body temperatures dipped during labor, only bouncing up again once the mini monkeys were out. The association of birth with a lower temperature could add weight to the statistics that show most primate babies arrive during the inactive period of the day, as the environment and core body temperature is lower at this time.

“Our findings suggest there may be important thermal consequences linked to the timing of primate birth,” wrote the authors in a statement. “Using state of the art bio-logging technology, we demonstrate that the timing of night-time births synchronise with both the maternal circadian rhythm, and the environmental conditions, that together maximise thermoregulatory efficiency during the birth process.”

Being primates, it’s very possible that monopolizing on thermal efficiency plays a role in patterns seen among human births, too. For humans, this would be at night but the lowering in body temperature is linked to circadian rhythm rather than the sun's position, so for nocturnal primates, their optimal birthing window is likely during the day when they are usually resting.

What time were you born? If you were a daytime surprise it seems you missed the memo, as a 2018 study found that 71.5 percent of spontaneous births happened in the evening and early morning hours – mostly between midnight and 6am.

Understanding why birthing patterns such as this exist goes beyond intrigue in that establishing the ideal conditions for childbirth as dictated by our evolution and physiology can get us closer to managing parent and baby safety during less optimal births.

“Our findings not only offer new insights into the thermal consequences of birth and the evolution of primate birth hour, but also may provide an evolutionary explanation for some of the health risks associated with human birth,” the authors concluded.