Mammalian embryos can enter a dormant state called diapause, which is crucial for developmental timing. In this study, researchers found that mouse embryos in diapause use lipids as their primary energy source. They discovered that supplementing embryos with the metabolite L-carnitine not only balances lipid consumption but also deepens the dormancy, extending embryo longevity up to 34 days in culture. The researchers identified FOXO1 as a key regulator of energy balance in dormant embryos and proposed that it may be a common regulator of dormancy across adult tissues. Their findings suggest that lipid metabolism is a critical metabolic transition relevant for longevity and stem cell function across tissues. The study highlights the importance of FOXO1 and lipid metabolism in maintaining a deeper dormant state, which is reversible and can be further explored for applications in stem cell research and regenerative medicine.Mammalian embryos can enter a dormant state called diapause, which is crucial for developmental timing. In this study, researchers found that mouse embryos in diapause use lipids as their primary energy source. They discovered that supplementing embryos with the metabolite L-carnitine not only balances lipid consumption but also deepens the dormancy, extending embryo longevity up to 34 days in culture. The researchers identified FOXO1 as a key regulator of energy balance in dormant embryos and proposed that it may be a common regulator of dormancy across adult tissues. Their findings suggest that lipid metabolism is a critical metabolic transition relevant for longevity and stem cell function across tissues. The study highlights the importance of FOXO1 and lipid metabolism in maintaining a deeper dormant state, which is reversible and can be further explored for applications in stem cell research and regenerative medicine.