Fetal sleep: a cross-species review of physiology, measurement, and classification

Clinical Snapshot

Key Findings

• P Value: Not applicable - narrative review

• Effect Size: Sleep cycling appears at 80-90% gestation in sheep/baboons vs 95% gestation in humans

• Primary Outcome: Fetal sleep state organization emerges at different gestational ages across species

• Nnt Or Sensitivity: Developmental milestone: organized sleep cycling as marker of neural circuit maturation

• Confidence Interval: Not applicable - narrative review

Clinical Application

Currently limited by lack of validated clinical frameworks; requires development of standardized non-invasive monitoring protocols Relevant to Australian obstetric practice through RANZCOG guidelines for fetal surveillance; potential integration with existing CTG monitoring protocols Pregnant women undergoing fetal monitoring, particularly high-risk pregnancies with suspected neurological compromise

Abstract

STUDY OBJECTIVES: Fetal sleep is a vital yet underexplored aspect of prenatal neurodevelopment. Its cyclic organization reflects the maturation of central neural circuits, and disturbances in these patterns may offer some of the earliest detectable signs of neurological compromise. This is the first review to integrate more than seven decades of research into a unified, cross-species synthesis of fetal sleep. We examine: (1) Physiology and Ontogeny-comparing human fetuses with animal models; and (2) Methodological Evolution-transitioning from invasive neurophysiology to non-invasive monitoring and deep learning frameworks. METHODS: A structured narrative synthesis was guided by a systematic literature search across four databases (PubMed, Scopus, IEEE Xplore, and Google Scholar). From 2925 identified records, 169 studies involving fetal sleep-related physiology, sleep-state classification, or signal-based monitoring were included in this review. RESULTS: Across the 169 studies, fetal sleep states become clearly observable as the brain matures. In fetal sheep and baboons, organized cycling between active and quiet sleep emerges at approximately 80%-90% gestation. In humans, this differentiation occurs later, around 95% gestation, with full maturation reached near term. Despite extensive animal research, no unified, clinically validated framework exists for defining fetal sleep states, limiting translation into routine obstetric practice. CONCLUSIONS: By integrating evidence across species, methodologies, and clinical contexts, this review provides the scientific foundation for developing objective, multimodal, and non-invasive fetal sleep monitoring technologies-tools that may ultimately support earlier detection of neurological compromise and guide timely prenatal intervention.

References

1. 1.Tang, W., Vargas-Calixto, J., Katebi, N., Galinsky, R., Clifford, G. D., & Marzbanrad, F. (2026). Fetal sleep: a cross-species review of physiology, measurement, and classification. Sleep. https://doi.org/10.1093/sleep/zsag003