Newborns have thermoregulatory mechanisms that differ from those of adults. Instead of producing heat through shivering, newborns primarily rely on non-shivering thermogenesis by the brown adipose tissue. The development of this thermogenic tissue starts around the 26th gestational week and continues until shortly before birth, after which no further growth occurs. As a result, premature infants, who have less developed brown fat, are more prone to reduced heat production and are at higher risk for hypothermia. There are few human studies examining the thermoregulatory differences and mechanisms between full-term and premature neonates, and the findings remain inconclusive. In this study, the investigators aim to conduct a prospective, observational research. Researchers will compare body temperature, brown adipose tissue activity, and specific plasma markers between full-term and premature neonates in insensive care units and during elective surgeries.