Some news reports have suggested that smartwatches or other wearable devices can detect signs of COVID-19 infection before it becomes symptomatic. If true, this could be a game-changer for managing the pandemic: People who know they are infected could begin to quarantine even before they develop symptoms, and thereby reduce community spread of the virus. Positive findings have been reported with multiple devices, including the Apple Watch , Fitbit (also here ) or Garmin wrist trackers, and the Oura Health smart ring . Primarily these studies have looked for changes in people's heart rate, although some of the analyses also include activity data and sleep patterns. A noted challenge of this and other chronic-disease-detection studies is that heart rate, activity, and sleep can be quite variable within individuals over time, even under normal conditions. Think about the last time you took a trip on an airplane across time zones, for instance -- a certain level of...