Why is there a red bar / section / block in my sleep graph? - Sleep as Android

The red block indicates that something went wrong with tracking at that time and the device stopped providing sensor data for some reason. Usually those are some non-standard battery optimizations or battery savers, the battery gets too low so we preserve it for the alarm or connectivity issue if you use a wearable.

This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org/faqs/red_sections_graph.html