By default, a document in view mode stays awake for 10 minutes after last viewed. While in view mode, results are cached so do not need to be recomputed. Once it returns to sleep mode, the results need to be re-computed. However, if you modify the sleep settings (click on the cog at the top-right and select Document Settings > Properties, you can delay how long it takes to go back to sleep mode. Increasing this time will use your Server time for private dashboards and API hours (view hours) at a faster rate.
You can also schedule times for the document to wake automatically. See Automatically Updating Calculations, R Variables, and R Data Sets.