In one embodiment, a computing device receives from a mobile device sensor signals which indicate motion associated with activities of a user of the mobile device. The computing device determines a number of activity types that correspond to at least a portion of the received sensor signals. The computing device determines a number of activity periods of the user, each activity period corresponding to a likely activity type, by analyzing the sensor signals against each of the activity types, and calculating a probability of each of the activity types. The computing device determines whether two or more of the determined activity periods should be combined into a single period, based on a heuristics-type analysis based on a length of time for each of the two or more activity periods. The activity periods are sent to the mobile device.