Heat causes expansion, and the heat of a human body will cause the internal mechanism of a wristwatch to expand. If there is a defect in the watch's electronic circuitry, that expansion may cause a circuit connection to break, which causes the watch to stop.
Watch circuitry is very delicate and can easily be broken by rough handling of the watch. Do not expose a watch to extreme temperatures, electricity, water, household chemicals or small particulates such as sand or soil. If a watch has stopped due to a broken circuit connection, it can be repaired by a professional watchmaker. Many watch manufacturers will repair or replace broken watches if they are under warranty. To get the best service from a timepiece, it's best to keep a watch clean and dry, treating it carefully and having it professionally serviced on a regular basis. While some people believe that the electrical field generated by a person can affect small electronics such as watches, there is no scientific evidence that this is true. If a watch often stops, consider replacing it with a watch designed for sports, since sports watches are often designed to stand up to sweat, shocks and heat fluctuations.