Fetal day-night rhythms in the womb predict infant sleep patterns

Long before birth, the human circadian clock begins to oscillate, and a new study suggests that the sleep preferences a fetus establishes in the womb may carry over into infancy. Researchers report that fetuses showing a stronger preference for nighttime sleep tended to maintain that preference after birth, pointing to a possible prenatal origin of early sleep behavior.

The study, published in Nature Communications, tracked fetal activity using continuous monitoring during the third trimester of pregnancy. Each recording lasted at least five days, a substantial extension over the short ultrasound snapshots that have historically been used to describe fetal behavioral states. The analysis included 32 mother-child pairs and generated more than 20,000 combined hours of wearable sensor data.

The sensors captured maternal acceleration and temperature alongside fetal acceleration, allowing the team to examine how fetal activity aligned with the day-night cycle, a pattern known as a diurnal rhythm. The authors note that such alignment had remained poorly understood precisely because long-term fetal monitoring has been lacking.

Using multi-level modeling, the researchers identified diurnal rhythms in the fetuses that appeared to align with maternal activity and circadian rhythms. This suggests that the developing fetal clock may be synchronized with signals from the mother, although the observational design of the study cannot establish the mechanisms behind that alignment.

To probe whether these prenatal patterns matter after birth, the team followed the infants to 6 months of age and assessed sleep at three postnatal time points: the first weeks, 3 months, and 6 months. Sleep was measured with sleep diaries and questionnaires completed by the parents.

A machine learning approach, Random Forest analysis, was used to identify which prenatal factors best predicted postnatal infant sleep. The strongest predictor was the fetal day/night sleep duration ratio, described by the authors as a simplified proxy for fetal circadian entrainment. Fetuses that preferred nighttime sleep maintained that preference after birth, according to the study.

Maternal behavior during pregnancy also appeared to leave a mark. Maternal sleep regularity, defined as consistent sleep patterns characterized by low day-to-day variability in sleep timing, predicted infant sleep as well. Specifically, more regular maternal sleep was associated with a stronger preference for nighttime sleep in the infants.

The authors highlight maternal sleep as an actionable target that could potentially improve infant sleep and family well-being, framing the findings as evidence that intrauterine and maternal factors may influence early circadian entrainment. However, they caution that the cohort of 32 mother-child pairs is relatively small, and they advise that the findings should be interpreted with caution until they are replicated in larger, independent cohorts.

The study also tracked diurnal pattern development continuously from in utero to 6 months of age, offering a rare longitudinal view of how circadian rhythms emerge across the transition from fetal to infant life. Because the article is being shared early as a peer-reviewed, accepted manuscript subject to further edits, some details may change in the final version of record.

Subject of Research: Prenatal fetal activity rhythms and their association with postnatal infant sleep behavior

Article Title: Association of fetal activity patterns in utero with postnatal sleep behavior

Article References: Markovic, A., Blume, C., Mühlematter, C., Zimmermann, P., & Kurth, S. (2026). Association of fetal activity patterns in utero with postnatal sleep behavior. Nature Communications. https://doi.org/10.1038/s41467-026-78754-9

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78754-9

Keywords: fetal development, circadian rhythms, infant sleep, pregnancy, wearable sensors, diurnal rhythm, maternal sleep, sleep regularity, third trimester, Nature Communications, Association, fetal