1. A method comprising: receiving heartbeat data of a patient; receiving activity data of the patient, wherein the activity data includes an activity value related to an activity level of the patient that is measured independently of the heartbeat data; determining one of a plurality of activity value ranges to which the activity value belongs; determining a weighting factor based on the determined activity value range, each of the plurality of activity value ranges associated with a different one of a plurality of predetermined weighting factors; determining modified heartbeat data by applying the weighting factor to at least a portion of the heartbeat data; and detecting a seizure event based on the modified heartbeat data.