Fitbit never recommends wearing any of its fitness trackers on the ankle. The pulse rate is one of the important parameters that it takes into account for many of its highlighted features. Sleep tracking is one of them. Depending on your heartbeat and breathing, it analyses and figures out your sleep pattern. So wearing any of the fitness bands on the ankle will simply work against it as the readings will be spurious and simply garbage. So, to answer the question, Fitbit cannot track sleep when worn on the ankle.