Humans and most other organisms have an internal biological clock that tracks the daily cycle of sunrise and sunset. These clocks help time sleep, metabolism, and other important bodily functions throughout the day, creating daily patterns called circadian rhythms. Research shows that when these rhythms are disrupted by things like jet lag, sleep deprivation, and irregular work schedules, people can experience negative long-term health effects.
Scientists who study rhythms of life have long wondered when mammals’ circadian clocks start ticking and synchronize with local time. In a new study published in biological rhythm journal, Researchers at Washington University in St. Louis have reported that mothers help adjust their babies’ biological clocks while they are still in the womb.
Disruption of circadian rhythms during pregnancy can affect the infant’s sleep and rhythm development, and these early disruptions are known to increase the risk of mood disorders such as anxiety and depression later in life. Understanding when the fetal clock begins to function can help identify when disruptions to circadian rhythms can have lasting effects and how those effects can be prevented or modified. ”
Nikhil Lokesh, study author, biology researcher at WashU Arts & Sciences
For this new study, WashU scientists developed a method to observe circadian clock activity in fetuses still developing in the womb. The researchers used genetically engineered mice in which a light-emitting protein called luciferase, the same protein that makes fireflies glow, was linked to a clock protein that drives circadian rhythms.
When male mice carrying this modified protein are mated with normal females, the tagged clock protein appears in the developing fetus but not in the mother’s tissues. The pregnant mice were then given drinking water mixed with a chemical that reacts with luciferase to produce light. Every time the clock protein was active inside the fetus, the fetus lit up.
The researchers used a highly sensitive camera to detect the light in the fetus. By recording the timing of light, researchers were able to identify distinct periodic patterns of clock protein expression in babies developing in their mothers’ wombs.
“We discovered a distinct day/night rhythm in the pups that is synchronized with the mother’s cycle of rest and activity during the last week of pregnancy, which corresponds to the third trimester in humans,” Lokesh said. “This suggests that the clock mechanism is formed early in development and then receives entrainment cues from the mother.”
“Importantly, we found daily rhythms across the placenta, from mother to baby, before the fetus can detect light,” said Eric Herzog, lead author of the study and Victor Hamburger Distinguished Professor of Biology.
The researchers found that the circadian synchronization of pups and their mothers coincides with when glucocorticoid hormones from the mother cross the placenta, potentially acting as a timing signal for the fetal clock. These stress-related hormones typically rise and fall throughout the day under the control of the mother’s biological clock.
Synthetic glucocorticoids are routinely given to pregnant women at risk for preterm delivery, often without taking into account the time of day that these hormones naturally fluctuate. The authors found that giving these steroids daily to the mothers helped synchronize the pups’ rhythms to local time. These findings can be important when doctors consider when and how to administer drugs to treat pregnancy symptoms.
During the study, researchers also observed a strong association between failure to develop circadian clock gene activity in the fetus and failure to give birth. “We can’t yet say whether the lack of rhythm is contributing to the developmental problem or just reflecting a developmental problem,” Lokesh said. “However, this observation suggests that circadian clock activity may be closely linked to healthy fetal development.”
Lokesh said the findings also highlight the importance of maintaining stable circadian rhythms during pregnancy. “More than 80% of the world’s population is exposed to artificial light that can disrupt their daily rhythms at night, including pregnant people,” he said.
“Understanding when and how the body clock starts ticking can help scientists identify sensitive developmental periods when disruptions to circadian rhythms can have lasting effects,” Lokesh said. “This knowledge could help guide medical care, inform clinical practice, and shape public health policies aimed at protecting the circadian health of newborns during pregnancy.”
sauce:
Washington University in St. Louis
Reference magazines:
Nikhil, KL, et al. (2026). Fetoplacental circadian rhythms develop and then synchronize with the mother in utero. biological rhythm journal. DOI: 10.1177/07487304261435435. https://journals.sagepub.com/doi/10.1177/07487304261435435
