This study reveals that FOXO1-mediated lipid metabolism is essential for maintaining mammalian embryos in a dormant state called diapause. In vitro, supplementing embryos with the metabolite L-carnitine enhances lipid consumption, deepens dormancy, and extends embryo survival. The research identifies FOXO1 as a key regulator of energy balance in dormant embryos and suggests it may play a common role in maintaining dormancy across adult tissues. Diapause is a reversible non-proliferative state that helps preserve early embryos under certain conditions. It is also a key feature of stem cell function. The mTOR pathway regulates dormancy by adjusting cellular growth and metabolism. Inhibiting mTOR (mTORi) induces a diapause-like state in embryonic stem cells and mouse pre-implantation embryos. However, significant embryo loss occurs over time. The inner cell mass (ICM) and trophectoderm (TE) respond differently to dormancy cues. The study shows that ESCs can reversibly pause in response to mTORi, while TSCs cannot. Time-resolved proteomics analysis reveals distinct pathways critical for establishing a paused state. The metabolic switch to fatty acid oxidation (FAO) is necessary for dormancy maintenance, with FOXO1 and L-carnitine as essential regulators. Supplementing embryos with L-carnitine extends their survival up to 34 days in culture. The study proposes that L-carnitine enhances in vitro diapause by establishing a deeper dormant state and highlights the FOXO1/FAO axis as a regulator of dormancy in adult tissues. The research also shows that lipid metabolism is crucial for maintaining the balance between stem cell dormancy and proliferation in various adult tissues and during embryonic diapause. The findings suggest that lipid metabolism may be a critical metabolic transition relevant for longevity and stem cell function across tissues. The study provides insights into the cellular mechanisms of dormancy entry and maintenance, and the role of FOXO1 in regulating energy balance and dormancy.This study reveals that FOXO1-mediated lipid metabolism is essential for maintaining mammalian embryos in a dormant state called diapause. In vitro, supplementing embryos with the metabolite L-carnitine enhances lipid consumption, deepens dormancy, and extends embryo survival. The research identifies FOXO1 as a key regulator of energy balance in dormant embryos and suggests it may play a common role in maintaining dormancy across adult tissues. Diapause is a reversible non-proliferative state that helps preserve early embryos under certain conditions. It is also a key feature of stem cell function. The mTOR pathway regulates dormancy by adjusting cellular growth and metabolism. Inhibiting mTOR (mTORi) induces a diapause-like state in embryonic stem cells and mouse pre-implantation embryos. However, significant embryo loss occurs over time. The inner cell mass (ICM) and trophectoderm (TE) respond differently to dormancy cues. The study shows that ESCs can reversibly pause in response to mTORi, while TSCs cannot. Time-resolved proteomics analysis reveals distinct pathways critical for establishing a paused state. The metabolic switch to fatty acid oxidation (FAO) is necessary for dormancy maintenance, with FOXO1 and L-carnitine as essential regulators. Supplementing embryos with L-carnitine extends their survival up to 34 days in culture. The study proposes that L-carnitine enhances in vitro diapause by establishing a deeper dormant state and highlights the FOXO1/FAO axis as a regulator of dormancy in adult tissues. The research also shows that lipid metabolism is crucial for maintaining the balance between stem cell dormancy and proliferation in various adult tissues and during embryonic diapause. The findings suggest that lipid metabolism may be a critical metabolic transition relevant for longevity and stem cell function across tissues. The study provides insights into the cellular mechanisms of dormancy entry and maintenance, and the role of FOXO1 in regulating energy balance and dormancy.