Energy conservation in battery powered tag

Method and apparatus for conserving energy in a battery powered tag which comprises a battery, one or more sensors, and a transmitter. The tag may be used in a pneumatic tire monitoring system which senses and transmits tire pressure and temperature readings. The method fundamentally comprises: hibernating between transmissions by the transmitter; and reading at least one of the sensors after each of a plurality of sleep periods while hibernating. A hibernation period may be ended if one of the sensor readings, preferably tire temperature, is very low, and at least one of a plurality of qualification criteria is met.