Disclosed is a method of operating a low power wireless receiver in which a radio is periodically operable for receive intervals with sleep intervals therebetween and comprising a sleep clock having a sleep clock accuracy. A first transmission or packet is received. Based on a start moment of the first received packet, and an expected interval between packets, a nominal start moment is determined to start the radio for a packet window until a nominal end moment, for receiving a second packet; the packet window duration is extended in dependence on an estimated drift based on the SCA to provide a widened window. A start moment of a second received packet is measured within the widened window. An actual drift is calculated, from the start moment of the second packet; and an actual start moment and an actual window duration is determined, for receiving a third packet, based on the actual drift.