Despite their compensatory mechanisms, neonates, particularly low-birth-weight infants, have limited capacity to thermoregulate and are prone to decreased core temperature. Even before temperature decreases, cold stress occurs when heat loss requires an increase in metabolic heat production. The neutral thermal environment (thermoneutrality) is the optimal temperature zone for neonates; it is defined as the environmental temperature at which metabolic demands (and thus calorie expenditure) to maintain body temperature in the normal range (36.5 to 37.5° C rectal) are lowest. The neutral thermal environment has a narrow range from 36.7° to 37.3° C.