Users are typically active during a Challenge. But to help establish activity, there are automatic notifications that go to users when they're inactive. There are a variety of messages relayed, but they provide tips, encourage the user to try a specific Action, or just generally encourage activity.
These notifications are generated after each of the following periods of inactivity:
36 hours of inactivity (new user)
4 days of inactivity
15 days of inactivity
30 days of inactivity