It’s widely known that a good night’s sleep is a basic for our health and beauty, but a new study says that it can boost our metabolism too—if we sleep in cool rooms. “Just by sleeping in a colder room, they gained metabolic advantages,” Francesco S. Celi, M.D., lead author of the study, said of his five male test subjects. He found that lowering the temperature of your bedroom while you sleep may expand your brown fat. Now, we realize it’s a pretty gross term, but brown fat is actually the best thing ever for your metabolism. This good fat produces heat by burning energy to maintain your core temperature; when your body temperature drops, brown fat kicks in to keep you warm, burning calories in the process. We think of traditional fats as being our figure foes, but in the case of brown fat it’s the exact opposite: Higher amounts have been found in people with speedy metabolisms. The researchers found that after four weeks of sleeping in 66 degrees the volunteers presented twice the volume of brown fat. But the effect will last only as long as you sleep in a chilled room: After four weeks of sleeping at 81 degrees, the brown fat retreated and all those glorious metabolic enhancements were undone. So stop arguing over the right temperature, we’ve decided: it’s 66.