If its cloudy and/or raining during the middle of the night, it'll be close to pitch black.
Clear skies on the other hand you can easily see as long as your monitor's brightness is correctly calibrated.
The only way to fix this is to manually add lights for the correct lighting for each interior. He's planning on doing this, but it can only be done with the CK. In the mean time, some interiors will appear unusually dark due to lack of proper lighting (dependance on ambient light) inside and the fact that MANY interiors use the same exact image spaces as dungion interiors.