Before a baby sees its first sunrise, it is already receiving signals about day and night. Melatonin, a hormone released mainly at night, can pass from the mother through the placenta. The placenta supplies the developing foetus with oxygen and nutrients.

This may help explain interest in a Japanese study of 31,528 mothers and children. It found a link between working night shifts during pregnancy and possible developmental difficulties in children at age four. But it could not show that night work caused those difficulties.

Researchers compared women who did not work night shifts, women who stopped between two pregnancy surveys, and women who reported night shifts at both surveys. Working just one night shift a month counted as night work. The study did not examine whether more frequent shifts carried greater risks.

Children whose mothers reported night shifts at both stages had a 29% to 34% higher relative risk of being flagged for possible difficulties with communication, fine motor skills (such as picking up small objects) or social skills, compared with children whose mothers did not work nights.

That does not mean a third of these children had difficulties. Across all groups, about 13% of children were flagged for a possible difficulty in at least one area.

Parents answered questions about their children’s abilities. These were screening checks, not medical diagnoses. A flagged result means a child might need further assessment.

The study had limitations. Researchers did not know exactly how many nights the women worked, how demanding their jobs were or why some stopped working nights.

Researchers also looked at sleep duration. The link between night work and developmental screening results appeared stronger among women who reported sleeping at least ten hours. But only 80 women both continued working nights and reported that much sleep. This finding is uncertain and does not show that sleeping longer is harmful.

The study also cannot tell us whether stopping night shifts would reduce any risk.

How babies learn about day and night

One possible explanation involves the body’s internal clock. This controls daily patterns in sleep, hormones and other processes. Before birth, the internal clock is still developing, so signals from the mother may help tell day from night.

Night work can disturb this timing. Even when someone gets enough sleep, sleeping during the day and staying awake at night can put their routine out of step with their body clock. Light and meal times can affect it too.

Animal research offers clues about how these signals reach a foetus in the womb. In a 2025 rat study, researchers found that substances from the mother’s food rose and fell throughout the day in an area of the unborn rat’s brain that would later help control its body clock. This happened before the clock was fully working.

In a 2026 mouse study, researchers gave pregnant mice a stress-related hormone at different times of day. The timing changed daily rhythms in their developing offspring and the placenta.

These studies suggest that food-related substances and hormones may help pass information about time from mother to foetus. But they do not show whether this explains the Japanese findings or happens in the same way in humans.

What early light exposure can tell us

Other mouse studies have tested whether the amount of daylight during early life affects body clocks later on.

Mice raised under long, summer-like days developed different patterns of activity in their brain clocks from mice raised under short, winter-like days. Some differences remained after the lighting changed.

Another study exposed mice to different day lengths from pregnancy until weaning. Even after they were put on the same light and dark schedule, mice raised under longer days spent more time asleep during daylight hours.

These experiments suggest that early light patterns can have lasting effects on mice. But seasonal changes in daylight are very different from working night shifts, and the findings cannot tell us whether human babies are affected in the same way.

Timing signals may also continue after birth. Breast milk contains melatonin, and its levels change through the day. In a small 2026 trial involving 60 premature babies, those given expressed milk at the same time of day it had been collected spent longer in quiet sleep by the time they left hospital. The researchers did not look at longer-term development.

Scientists are still learning how babies receive signals about day and night. The Japanese study raises useful questions about supporting pregnant night workers, including whether more flexible working hours could help. But it cannot establish that disrupted body clocks caused developmental difficulties, and it gives parents no reason to fear that an occasional sleepless night will harm their child.