The study shows that the hormone ghrelin, which rises when you fast, can make your body temperature drop and put you into a hibernation‑like state called torpor. Mice that can’t make ghrelin never enter torpor and have trouble controlling their temperature. Ghrelin does this by turning down the nervous system signals that normally fire up brown fat, the tissue that burns calories to keep you warm.

Ghrelin, a circulating orexigenic hormone secreted from the stomach, stimulates appetite and food intake by activating the hypothalamic arcuate nucleus. Administration of exogenous ghrelin exerts anabolic effects, causing weight gain, increased adiposity, and decreased metabolism. Body temperature (BT), which is determined by the balance of heat production and heat loss, must be strictly regulated to maintain proper cellular function and metabolism. However, the role of ghrelin in thermoregulation remains unclear. In this study, we found that ghrelin was essential for decreasing BT when mice are placed under calorie restriction. Elevated ghrelin concentrations induced by fasting correlated with significant decreases in BT, a hibernation-like state called torpor. Ghrelin-deficient (Ghrl^-/-) animals could not enter torpor. The BT of Ghrl^-/- mice also remained high under restricted feeding, but the animals gradually entered precipitous hypothermia, indicating thermoregulatory impairment. These effects of ghrelin on thermoregulation were the result of suppression of sympathetic nervous system activity input to brown adipose tissue; in the absence of ghrelin, it was not possible to suppress uncoupling protein 1 (ucp1) expression and decrease BT in low-energy states. Together, these findings demonstrate that ghrelin is an essential circulating hormone involved in lowering BT.