Various folks have mentioned that even with "no sleep when powered" setting enabled, and running Fully constantly, etc., Amazon Fire tablets still seem to switch to the lock-screen overnight or some periodic basis.
Myself and Jeremy have noticed that there is an Alarm icon in the Notification Bar that coincides with the timing of the issue. Not sure this is always the case.
The problem is that the Alarm has no discernable source. Googling is inconclusive so far.
Customer support service by UserEcho