I've been stationed aboard the Navy base in Sasebo, Japan for about 3 months. Since the weather's nice, I usually sleep with my window open. My window faces the downtown area of the city. At least 3 times in the past months I've woken up to the city's sirens sounding a bastardized version of the "attack" tone at 3:15 am. Does anybody have any idea why a large city would sound their sirens at 3:15 am?
One time, the base sounded its sirens as well.
The city does a daily noon test.