That Silent Hunter IV World.....

[Seminole]

As far as the map changing, it is to simulate "rigging for red" at dusk and going back to normal lighting at dawn. It serves the purposes of (1) keeping night vision so if you have to go outside or look out without warning you won't be blind (can take 30 minutes or so to adjust vison to dark), and (2) keep some reference for crew as to night/day.

...Only problem with that theory is that the dawn,day, dusk,night lighting changes occur according to time shifts relative to your home base.

It does very little good to "rig for red" when it is broad daylight up on deck and it remains hours to nightfall at your current location.